[Feature] Double Tap Quick Access Splitter Bar should collapse editor area for Full Preview

If you double-click/tap the Quick Access bar:

_In middle or outside button areas._

Then it should collapse the editor area to show a full preview quickly:

_**Double-tapping again would restore the bar to its prior position.**_

Should work in all orientations.

Probably should store as new value in `LoadedDocument.State`, maybe an enum value as maybe there's alternatives for showing full editor or if preview is ever undockable...?

Not sure how this coordinates with the existing VSM as they'd both need to modify the grid properties, so may be incompatible with current layout approach...? Not as quick as it seemed at first to implement (hiding editor is easier, but positioning is not trivial, I think).