ResourceModel: implement filter functionality

So far all subclasses implemented filtering on their own which doesn't
make much sense. Now one can set filter keywords and a filter type for
all ResourceModels. Subclasses simply have to call
ResourceModel::itemMatchesFilter( ResourceItem ) in order to determine
whether an item should be hidden or not. Additionally they have to
implement the updateFilters() method in order to update their internal
management data each time the filters change.

src/gui/ResourceTreeView.cpp

@@ -48,11 +48,11 @@ void ResourceTreeView::setFilter( const QString & _s )
 	if( _s.isEmpty() )
 	{
 		collapseAll();
-		m_tm->setFilter( _s );
+		m_tm->setKeywordFilter( _s );
 	}
 	else
 	{
-		m_tm->setFilter( _s );
+		m_tm->setKeywordFilter( _s );
 		expandToDepth( _s.size() );
 	}
 	setUpdatesEnabled( true );