I mostly did it so I could move tile mode logic into a new node, so I could have control over the drawing order. TileMode.gd also always draws 8 blank rectangles, which are being drawn on top of the grid, in order to hide it if it gets out of boundaries. We are now very close to implementing an isometric grid, #305.
This shouldn't have much of a performance impact, but some more testing is most likely needed.
Instead of drawing individual lines, execute draw_isometric_tile() multiple times. What this method does is basically draws a single isometric tile, which then gets repeated for the entire canvas size. Also moved all the grid logic to a new script, which gets updated less often that Canvas.gd, for optimization purposes.
Works better than the previous method, although it might be a bit slower, and currently does not work well if the canvas size is not divisible by the grid size. For now, it only gets affected by the grid width. Changing grid height has no effect.
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.
ColorAndToolOptions has now the same code for left and right tool options, with more similar refactoring coming soon to places like Canvas and DrawingAlgos
