Update normative wording of 4.1.3 and definition of "status message"

[patrickhlauke]

The current normative wording for 4.1.3 Status Messages has ... issues:

• the mention of through role or properties would disallow the use of upcoming/new solutions, such as ariaNotify, since they are programmatic but do not rely on roles/properties
• the preface of In content implemented using markup languages limits the scope of this SC, which results in 4.1.3 not being applicable in cases where WCAG is being applied to other technologies (WCAG2ICT) - the current https://www.w3.org/TR/wcag2ict/#applying-sc-4-1-3-status-messages-to-non-web-documents-and-software notes that the SC "applies as written", meaning that in non-markup-language scenarios, it doesn't ... but then provides a note saying it sure would be nice if it did ("there is still a user need...") - and this handwave is also present in current draft versions of the upcoming EN 301 549 v4.1.0; many auditors currently "ignore" the markup language part when evaluating native mobile/desktop apps against WCAG, and take the fundamental principle - that AT must announce status messages when these don't receive focus - regardless

In addition, this PR tweaks the definition of what a "status message" is - there have been discussions (such as #4672) around the fact that the definition as it currently stands can actually apply to a lot more than just what was originally intended (e.g. if, after activating a control, a table is updated ... is the whole table a "status message"? based on the current definition, it can be seen as such). the update here tries to more specifically define status messages as discrete/explicit messages (excluding the "implicit status message" given by content, like a table, being updated). it also moves out the whole part about change of context - a status message is a status message, regardless of whether or not it changes context. this part about context change was already present in the normative wording of the SC, but has been reinforced.

I believe this change, while quite deep, is backwards-compatible with the existing/current version of the wording the way it was originally intended

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Current normative wording (SC + term)

Current normative wording of the SC, the definition, and "grafting" (with tweaks from singular to plural) that definition into the normative wording:

In content implemented using markup languages, status messages can be programmatically determined through role or properties such that they can be presented to the user by assistive technologies without receiving focus.

change in content that is not a change of context, and that provides information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors

In content implemented using markup languages, change[s] in content that [are] not a change of context, and that [provide] information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors can be programmatically determined through role or properties such that they can be presented to the user by assistive technologies without receiving focus.

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Proposed normative rewording (SC + term)

This PR proposes the following change to the SC, the definition, and shows the resulting "grafted" version (with tweaks from singular to plural) :

Status messages that do not cause a change of context can be programmatically determined such that they can be presented to the user by assistive technologies without receiving focus.

discrete piece of content that is added or updated dynamically, and whose only purpose is to provide information to the user on the results of an action, on the waiting state of an application, on the progress or completion of a process, or on the existence of errors

[D]iscrete piece[s] of content that [are] added or updated dynamically, and whose only purpose is to provide information to the user on the results of an action, on the waiting state of an application, on the progress or completion of a process, or on the existence of errors that do not cause a change of context can be programmatically determined such that they can be presented to the user by assistive technologies without receiving focus.

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Closes #4676

This PR supersedes #4840 (as that one originally only tried to update the definition, but further discussion on the PR made it clear that there's more fundamental work to be done, if we're already aiming for a normative change)

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