Clean up xpm small icons.
- Only constraint icons seem to be necessary
Mode Auto-switch to edge (Sponssi mode) has selection combo disabled to avoid interferring with this mode.
However, if auto-switch checkbox is unchecked (Jim-Abdullah mode), the combo is enabled and selection of
element type can be effected via the combo with the mouse.

In any case "z" shortcut iterates round robin the "valid types" of the preselected (hovered) element.