Highlight shapes and pixels on hover

[pklampros]

Allows for highlighting shapes and pixels when hovering over them in the GL view. Also provides an option to disable this in the settings. The effects for

single pixel:

multiple pixels:

multiple pixels while linking:

axial line:

multiple axial lines:

axial lines when linking:

polygon (convex maps):

multiple polygons (convex maps):

linking polygon (convex maps):

Option: