* Fix: Propagation of _gui_input()
_gui_input() calls only propagate up the scene tree for mouse events
with appropriate filter flags. Keyboard and controller events do not
propagate through _gui_input().
Places entire description of _gui_input() propagation in one location,
marks it as a note to separate it from surrounding input propagation,
and updates the description to include behavior of keyboard and
controller events.
1fe10df90d