Blender Git Loki

Change edge loop select and edge ring select to be consistent with
other selection operators. Instead of having only a single "extend"
option which causes the selection to toggle, there are now
three options:
- extend: adds to selection
- deselect: removes from selection
- toggle: toggles the selection

The default keymap is adjusted to have identical behavior as before,
using these new options, so end-users shouldn't experience any
difference.