Fix SIGSEGV when introducing a new shortcut (#9869)

Fix SIGSEGV when introducing a new shortcut (#9869)

removeShortcut() restores default settings, therefore was used incorrectly. I
introduce deactivateShortcuts() which only removes assignments.

Clean up a bit the lack of view / model distinction (getting rid of the crashing
code at the same time).

Repair inconsistency of the selection in the "modify" case. (regression at
717d19d3c)

Make the test for existing bindings a bit more robust. (Not perfect yet.)

Focus on the item that has just been added/modified. (cosmetic)