Improved Selection Functionality

[CaptainCooky]

Given that selections are arguably the cornerstone of any Editor Tool such as this, I suggest a few added capabilities that might improve their functionality.

1. Save/Load Selection - Similar to how you can do this in Photoshop. Having the ability to Save a Selection Configuration to load later in the Project or even across Projects would be a useful feature.

2. Remove from Selection Modifier - Once again, similar to Photoshop, having a modifier key to remove an area from a Selection without restarting the process would be very handy for those errant clicks we all occasionally make.
   With the ctrl key already assigned the "add-to" selection modifier, we need another one to "remove from" a selection.

3. Hide/Show Selection toggle - It might also be useful to have the ability to Hide/Show the Selection graphics.
   E.g. I'm finding that if I'm experimenting with Block palettes/Choices in an area and using the Copy/Paste tools, I can't easily assess whether I am happy with my choice without removing the Selection. Being able to Hide/Show the Selection without actually removing it would indeed be useful.

4. Negative Values in the Size field - If one were to use the "XYZ" values in the Size field of the Selection pane to change the size of a Selection Area, it is only possible to enter Positive Values. This results in the Selection only expanding in 1 direction. (Consistent for each Axis).
   e.g. If I were to select a block and then change the value in the "X" field from 1 to 5, the selection expands in the positive direction relative to the world coordinates. So, we could assume that entering a Negative Value would result in the Selection expanding in the opposite direction.
   I'm guessing that this is by design as a negative value in a Size field wouldn't make sense in the context of Selection Size, however maybe it could be implemented in such a way that entering a negative value would expand a Selection in the relevant direction, but then normalize to a Positive value after the change has been executed. (This is why I out this in a Feedback thread instead of Bug thread)