The previous implementation for signals mouse_entered and mouse_exited
had shortcomings that relate to focused windows and pressed mouse buttons.
For example a Control can be hovered by mouse, even if it is occluded by
an embedded window.
This patch changes the behavior, so that Control and Viewport send
their mouse-enter/exit-notifications based solely on mouse position,
visible area, and input restrictions and not on which window has
focus or which mouse buttons are pressed. This implicitly also
changes when the mouse_entered and mouse_exited signals are sent.
This functionality can not be implemented as a part of
Viewport::_gui_input_event, because of its interplay with Windows and
because Viewport::_gui_input_event is based on input and not on
visibility.
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