Rename SelectionRectangle to SelectionShape, make it have non-rectangular shape and multiple SelectionShapes can exist

- Create multiple selection rectangles
- Merge them together if they intersect
- Move the selections (without contents as of right now)
- Gizmos are being drawn but they are not functional yet

Code is very ugly.

project.godot

@@ -84,6 +84,16 @@ _global_script_classes=[ {
 "language": "GDScript",
 "path": "res://src/Classes/Project.gd"
 }, {
+"base": "Reference",
+"class": "Selection",
+"language": "GDScript",
+"path": "res://src/Classes/Selection.gd"
+}, {
+"base": "Polygon2D",
+"class": "SelectionShape",
+"language": "GDScript",
+"path": "res://src/Tools/SelectionShape.gd"
+}, {
 "base": "Guide",
 "class": "SymmetryGuide",
 "language": "GDScript",
@@ -105,6 +115,8 @@ _global_script_class_icons={
 "PaletteColor": "",
 "Patterns": "",
 "Project": "",
+"Selection": "",
+"SelectionShape": "",
 "SymmetryGuide": ""
 }