The default image width, height and fill color are being used only when the program first launches. After that, if the user creates an image with changed settings, these settings are being kept.
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.
