Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
* respect the 'auto enable otf' setting
* don't ask if otf is off or for layers without geometry
* ask when otf was switched on
* show more information in selection dialog
