Supervisor, Frame Pack and Misc Fixes

* 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.)

widget.go

@@ -1,6 +1,8 @@
 package ui
 
 import (
+	"fmt"
+
 	"git.kirsle.net/go/render"
 	"git.kirsle.net/go/ui/theme"
 )
@@ -252,6 +254,12 @@ func (w *BaseWidget) ResizeBy(v render.Rect) {
 
 // ResizeAuto sets the size of the widget but doesn't set the fixedSize flag.
 func (w *BaseWidget) ResizeAuto(v render.Rect) {
+	if w.ID() == "Frame<Window Body>" {
+		fmt.Printf("%s: ResizeAuto Called: %+v\n",
+			w.ID(),
+			v,
+		)
+	}
 	w.width = v.W
 	w.height = v.H
 }