Made a Frame class, no longer use multiple Canvases for multiple frames

The Canvas is now single node, instead of having multiple canvases for each frame. This should also be a performance optimization, since there are less canvas nodes, so there are less _input() calls. It should also fix a rare Undo/Redo issue with motion drawing and making lines. Could be unstable, needs more testing.

As a side effect, the guides are now the same for all frames, so this should also close #246.

src/Classes/Frame.gd

@@ -0,0 +1,9 @@
+class_name Frame extends Reference
+# A class for frame properties.
+# A frame is a collection of cels, for each layer.
+
+
+var cels : Array # An array of Cels
+
+func _init(_cels := []) -> void:
+	cels = _cels