Avoid deleting QtWebComboBox it's own nested QEventLoop

A race condition meant that sometimes a QtWebComboBox would be deleted
in an eventloop run while hiding itself. This patch adds a flags
and delays scheduling a deletion until after the nested eventloop.

Task-number: QTBUG-29844
Change-Id: Ia5e03954756afa29c0be0a2d7f49ae8d1dbba250
Reviewed-by: Michael Bruning <michael.bruning@digia.com>