Custom brushes, crop image, split screen, about menu & asset re-organizing

- Added support for custom brushes. When you Ctrl-C a selection, it gets added to the list of custom brushes. Each mouse button can have a different brush, and the user can choose whether their color comes from the brush itself or the selected color in the tool options. They can also be resized based on the selected brush size.
- Custom brushes are also being saved on .pxo files.
- You can now crop images (per frame). All layers of that frame are taken into account and are affected.
- Added split screen support. The user can toggle between single screen and split screen, where a second canvas is being shown. Note that you cannot draw on the second canvas.
- Added an About Pixelorama selection on the new Help menu.
- Project assets are re-organized.

Scripts/BrushButton.gd

@@ -0,0 +1,14 @@
+extends Button
+
+var brush_type = Global.BRUSH_TYPES.PIXEL
+var custom_brush_index := -1
+
+func _on_BrushButton_pressed() -> void:
+	if Input.is_action_just_released("left_mouse"):
+		Global.current_left_brush_type = brush_type
+		if custom_brush_index > -1:
+			Global.custom_left_brush_index = custom_brush_index
+	elif Input.is_action_just_released("right_mouse"):
+		Global.current_right_brush_type = brush_type
+		if custom_brush_index > -1:
+			Global.custom_right_brush_index = custom_brush_index