1. Add methods for coordinate translation between cairo/GtkWidget
coordinates and LdDocument coordinates.
2. Draw the grid correctly (also make it significantly faster).
3. Place the canvas into a GtkScrolledWindow and register
the set_scroll_adjustments signal in the GtkWidget base class.
4. Allow scrolling inside of an area of 200 x 200 units (for now).
In the future, it should be possible to go to infinity.