ResourceBrowser: move implementation of default actions to ResourceAction

Again there's no need for duplicated functionality. Adding a
ResourceAction::defaultTrigger() method allows to separate all
functionality from ResourceBrowser and possibly use it in other places
as well.

include/ResourceAction.h

@@ -59,6 +59,8 @@ public:
 	bool loadByPlugin( InstrumentTrack * _target );
 	bool loadPreset( InstrumentTrack * _target );
 	bool importProject( trackContainer * _target );
+	// most actions can be triggered without any further information
+	bool defaultTrigger();
 
 
 private: