* Fix Supervisor event issues wrt. the window manager feature: if a
focused window exists and Supervisor is running events for the "other"
widgets not in managed windows, and the mouse cursor is over the
rectangle of THE focused window, no widget under the cursor receives
active (hover, click) events. Prevents being able to click "through"
the window and interact with widgets and other windows below.
* Adds Close, Maximize and Minimize buttons to windows. Maximize is
still buggy and Minimize is implementation-defined behavior with no
default event handler configured.
* eg/windows has an example of the Window Manager for SDL2 and