Pattern: add undo/redo support for "Clear all notes" operation in context menu

Simply add a journal checkpoint before clearing pattern.

src/tracks/pattern.cpp

@@ -422,6 +422,7 @@ void pattern::loadSettings( const QDomElement & _this )
 
 void pattern::clear()
 {
+	addJournalCheckPoint();
 	clearNotes();
 	ensureBeatNotes();
 }