Toggleable grids

[serin-delaunay]

Wangview should be able to display grids aligned with the logical tiles and the graphical tiles. One or two keys should allow the user to switch between modes.

[serin-delaunay]

With rectangular tiles this can probably be done with unicode characters. Once other tile geometries are supported (Wangscape/Wangscape#32), this will have to be done using an extra set of output tiles.