Synchronize selection between the icon view and the thumbnail bar.
This allows multiple selection in the thumbnail bar with a meaning: all actions
are carried out as if selected in the normal icon view.

Drop using the imageActivated signal of thumbbar (with the same as clicked() anyway),
simply use the current image.

This also fixes the problem that the arrow keys in the thumbbar would move
the selection, but not update the shown preview (may be a bug report somewhere)