[FancyZones] Rework grid editor (#10116)

* Started rewriting

* Making progress

* Fix resizers not moving around

* Implemented splitting, fixed some bugs

* Removed more code, renamed methods

* Merging zones works

* Fix Shift key behavior

* Added spacing (has bugs)

* Implement minimum size restriction

* Match preview and editor visuals

* Snapping works

* Show when splitting is not possible

* Fix spell checker complaining

* Tweak FZ Lib function computing grid zones

* Fix potential crash when loading old zone layouts

* Fix dead objects talking

* Fix splitters being shown when they shouldn't be

* Fix index numbering

* Fix small glitch with the shift key

* Do not snap to borders outside the zone

src/modules/fancyzones/editor/FancyZonesEditor/Models/GridLayoutModel.cs

@@ -120,11 +120,6 @@ namespace FancyZonesEditor.Models
 
         private int _spacing = LayoutSettings.DefaultSpacing;
 
-        // FreeZones (not persisted) - used to keep track of child indices that are no longer in use in the CellChildMap,
-        //  making them candidates for re-use when it's needed to add another child
-        //  TODO: do I need FreeZones on the data model?  - I think I do
-        public IList<int> FreeZones { get; } = new List<int>();
-
         public GridLayoutModel()
             : base()
         {