Add KLineEditEventHandler
This unifies several event filter objects we have here and elsewhere for tweaking QLineEdit behavior.
- Return key catching to avoid closing dialogs (several copies, 100+ uses all over)
- URL drop handling (preexisting here)
Compared to exposing the event filter objects, this approach hides that implementation detail (we might for example merge the various filter objects one way or the other in the future), and avoids the consumer having to deal with memory management.