Add "Check for updates" to Electron menus, in-app menus, command palette

[franknoirot]

We should have an OS menu option to "check for updates" or that already knows if there is an update and executes it. This may not be the first place users look, such as Anand who checked the command palette here, so consider placing other, in-app ways to get to this.

Related to #3872, these buttons might open that updater dialog once it is re-established.