By default, Tooltips will present after their associated widget presents
(if the mouse cursor is hovering over that widget, and the tooltip
should appear). But the tooltip is not guaranteed to draw "on top" of
neighboring doodads, unless you choose your Edge carefully depending on
the order you're drawing your widgets.
To solve this, Tooltips can be supervised to DrawOnTop() when they're
activated. To opt in, you simply call the Tooltip.Supervise() function
with your supervisor.
* Adds Window Manager support to the Supervisor, so that Window widgets
can be dragged by their title bar, clicked to focus, etc.
* Create a ui.Window as normal, but instead of Packing or Placing it
into a parent container as before, you call .Supervise() and give it
your Supervisor. The window registers itself to be managed and drawn
by the Supervisor itself.
* Supervisor manages the focused window order using a doubly linked
list. When a window takes focus it moves to the top of the list.
Widgets in the active window take event priority.
* Extended DragDrop API to support holding a widget pointer in the drag
operation.
* Changed widget event Handle functions to return an error: so that they
could return ErrStopPropagation to prevent events going to more
widgets once handled (for important events).
Some bugs remain around overlapping windows and event propagation.
* 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.
