PianoRoll, Pattern: manage reset of currently edited pattern more reliably

Introduce one central signal in the pattern class which is emitted before
the pattern object is being destroyed. This way we can easily and more
reliably hide this pattern in the PianoRoll without any race conditions
or other glitches.

Closes #1089.

include/PianoRoll.h

@@ -164,6 +164,9 @@ protected slots:
 	void changeNoteEditMode( int i );
 	void markSemiTone( int i );
 
+	void hidePattern( pattern* p );
+
+
 signals:
 	void currentPatternChanged();
 	void semiToneMarkerMenuScaleSetEnabled(bool);

include/pattern.h

@@ -140,6 +140,10 @@ private:
 	friend class patternView;
 	friend class bbEditor;
 
+
+signals:
+	void destroyedPattern( pattern* );
+
 } ;