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.
Replaces nested Global.animation_tags arrays. Also replaced array.duplicate(true) with looping through the array and creating a new class for each array element, because duplicate(true) does not create new classes, unfortunately, which was causing issues with undo/redo.
Moved code from Global.gd and Canvas.gd to DrawingAlgos.gd. Will also move the fill_gaps and draw_brush methods of Canvas.gd next. Maybe even refactor the inside of them a bit to make them easier to read.
Removed separators entirely. Also made the AnimationTimeline have a StyleBoxFlat of its own, which we use to expand its margin top to cover the little space left because of the VSplitContainer. We also use a TextureRect as a fake VSplitContainer grabber.
