* 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
WebAssembly targets.
* Tooltip can be added to any target widget (e.g. Button) and pop up on
mouse over.
* Refactor the event system. Instead of passing a render.Point to all
event handlers, pass an EventData struct which can hold the Point or
the render.Engine.
* Add event types Computed and Present, so a widget can set a handler on
whenever its Computed or Present method is called.
* Button: do not call MoveTo inside of Compute().
* Label: do not call MoveTo inside of Compute().
* MainWindow: add OnLoop callback function support so you can run custom code
each loop and react to the event.State before the UI updates.
* Supervisor: locate widgets using AbsolutePosition() instead of w.Point()
to play nice with Frame and Window packed widgets.
* Widget interface: rename Adopt() to SetParent() which makes more sense for
what the function actually does.
* Window: set itself as the parent of the body Frame so that the Supervisor
can locate widgets anywhere inside a window's frames.
Frame packing fixes:
* Widgets with Expand:true grow their space with ResizeAuto to preserve the
FixedSize() boolean, instead of being hard-resized to fill the Frame.
* Widgets that Fill their space are resized with ResizeAuto too.
Outstanding bugs:
* Labels don't expand (window title bars, etc.)
