2D editor GUI input rework. Changes are:

- The input handling is done into several distinct functions, and the
  code is more consistent.
- The actions' history is more precise ("Edited CanvasItem"
  is now "Rotated CanvasItem","Moved CanvasItem",etc...)
- Fixed a little bug about input key events not forwarded correctly to plugins
- IK is followed by default when you move a bone node, the alt-key allow
  you to move it normally

scene/gui/base_button.cpp

@@ -307,6 +307,8 @@ void BaseButton::toggled(bool p_pressed) {
 }
 
 void BaseButton::set_disabled(bool p_disabled) {
+	if (status.disabled == p_disabled)
+		return;
 
 	status.disabled = p_disabled;
 	update();