Instead of having a frame_duration[] array in the Project class. This makes the code much more readable and understandable, using less lines of code and, by avoiding an extra array, we also avoid potential out-of-bound array crashes.
The functionality for the user has not changed, and by testing so far I didn't find any issues.
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.
