* Remove hint system on buttons. Buttons show currently selected
reference type, instead.
* Change button's caption to "Selecting..." when selecting.
* internal logic rearrangements
* adapt
* introduce mode selection
* kill selection gate to allow selecting whole objects (selection gate
should be reintroduced to disallow references between parts, no
restrictions is a temporary state)
* prevent auto-switchover to next ref after an explicit push of a button
* fix highlighting of buttons when selecting corresponding reference
* allow creation of a feature, even if no mode fits references. With a
warning.
