Improve 2D editor zoom logic

- Add 1-5 shortcuts to zoom between 100% and 1600% quickly
  (similar to GIMP).
- When holding down Alt, go through integer zoom values if above 100%
  or fractional zoom values with integer denominators if below 100%
  (50%, ~33.3%, 25%, …).

editor/editor_zoom_widget.h

@@ -56,7 +56,7 @@ public:
 
 	float get_zoom();
 	void set_zoom(float p_zoom);
-	void set_zoom_by_increments(int p_increment_count);
+	void set_zoom_by_increments(int p_increment_count, bool p_integer_only = false);
 };
 
 #endif // EDITOR_ZOOM_WIDGET_H