Utils: Introduce CompletingLineEdit

* Works around QTCREATORBUG-9453
* Triggers completion on Key_Down with empty prefix
* Replace current filters in HistoryCompleter and FancyLineEdit

Change-Id: I56bfd4e0ee969c5ae674de2f2de1081fcf6dc176
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Git: add completion to ChangeSelectionDialog

Change-Id: I4511c26af7b02fb0ffab76e5170e80d6cf79b10b
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>