PianoRoll: fix getKey and remove dead code (#8008)

- Fixes: when right clicking on a piano key close to the lower edge of the screen, "Mark semitone" will mark the wrong key because it uses the upper edge of the context menu as reference.
- Fixes: getKey() is off by 2 pixels
- Introduces: yCoordOfKey() which is the reverse of getKey()
- Removes: section of code that can never be reached

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Co-authored-by: Johannes Lorenz <1042576+JohannesLorenz@users.noreply.github.com>

include/PianoRoll.h

@@ -188,7 +188,6 @@ protected:
 	void focusOutEvent( QFocusEvent * ) override;
 	void focusInEvent( QFocusEvent * ) override;
 
-	int getKey( int y ) const;
 	void drawNoteRect( QPainter & p, int x, int y,
 					int  width, const Note * n, const QColor & noteCol, const QColor & noteTextColor,
 					const QColor & selCol, const int noteOpc, const bool borderless, bool drawNoteName );
@@ -339,6 +338,9 @@ private:
 	int noteEditRight() const;
 	int noteEditLeft() const;
 
+	int getKey(int y) const;
+	int yCoordOfKey(int key) const;
+
 	void dragNotes(int x, int y, bool alt, bool shift, bool ctrl);
 
 	static const int cm_scrollAmtHoriz = 10;