Follow-up 8dfbc5baa: Really don't select unselectable items

SelectWidget would refuse to select unselectable items on mousedown or
mousemove, but would happily do so on mouseup. This meant unselectable
items weren't actually unselectable.

This broke the link inspector: if you clicked on the text
"External link", for instance (a ui.MenuSectionItemWidget), that "item"
would be selected, the link target input would be cleared, if you exited
the link inspector in any way the link would be unlinked, and any new
link inspector you opened would have a completely non-functional dropdown.

Change-Id: I7faa3d23b51b9cb93e68414584b9f433ea1f656a
(cherry picked from commit 4721367b618a9c62f7cdc1385d02e94360606353)