A Project class contains project-specific data like name, undo_redo, frames, layers, tags and brushes. These variables have been moved from Global. This is the first step towards multiple tab support, where each tab will be a different Project.
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.
