Only show Piano Roll when editable (#6295)

- Display a message when a user attempts to open the piano roll in a project with no MIDI clips instead of opening a non-functional piano roll.
- Create a MIDI clip in the first instrument track when the piano roll is opened in an empty project.

include/PianoRoll.h

@@ -146,6 +146,7 @@ public:
 		return m_midiClip;
 	}
 
+	// TODO remove this since PianoRoll should no longer be visible without a valid midi clip
 	bool hasValidMidiClip() const
 	{
 		return m_midiClip != nullptr;
@@ -588,6 +589,8 @@ private slots:
 private:
 	void clipRenamed();
 	void focusInEvent(QFocusEvent * event) override;
+	void showEvent(QShowEvent* se) override;
+
 	void stopStepRecording();
 	void updateStepRecordingIcon();