Port overlapping link hint rotation by Space from VimFx

This commit basically ports the VimFx'es ability to rotate overlapping marker stacks by pressing Space (backward-rotation by Shift-Space is not ported as -- at least on my side -- pressing Shift causes the hints to disappear, may need to think of another shortcut if necessary). This is very useful in the cases where many little (e.g. iconic) links are stuffed together. Resolves #757.