Preferences: Fix shown widget when filtering

* choose an enabled tab if current tab gets disabled (for some reason
that is not automatically done by QTabWidget
* don't show any widget if the filter string is not matched by anything

Task-number: QTCREATORBUG-8318
Change-Id: I0066c2b3aeb9cc0cdacaa0a88f9adbc70c20cfa7
Reviewed-by: Daniel Teske <daniel.teske@digia.com>