UI: Add LineEditChanged and LineEditCanceled

These functions allow item delegates (editors) or item widgets with
event filters to detect whether a user has finished editing a line edit
control.  This separates the code so it can be used elsewhere than just
in the source tree widget.

UI/qt-wrappers.hpp

@@ -102,3 +102,6 @@ static inline Qt::ConnectionType WaitConnection()
 		       ? Qt::DirectConnection
 		       : Qt::BlockingQueuedConnection;
 }
+
+bool LineEditCanceled(QEvent *event);
+bool LineEditChanged(QEvent *event);