Add possibility for filters to set a new search text when accepting
an entry.
Move placeholder text update from locator manager to widget.
Propagate filter update through signal-slot connection instead of directly.
LocatorManager::show is the only place left that directly references the locator widget.

Change-Id: Id61354d9f166c2af8c9d5528ad8998c7c6b8e1ab
Reviewed-by: David Schulz <david.schulz@qt.io>