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.
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.
