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

@@ -1,8 +1,6 @@
 package ui
 
 import (
-	"fmt"
-
 	"git.kirsle.net/go/render"
 	"git.kirsle.net/go/ui/theme"
 )
@@ -68,7 +66,7 @@ type Widget interface {
 	// child widgets and the parent.
 	Parent() (parent Widget, ok bool)
 	SetParent(parent Widget) // for the container to assign itself the parent
-	Children() []Widget  // for containers to return their children
+	Children() []Widget      // for containers to return their children
 
 	// Run any render computations; by the end the widget must know its
 	// Width and Height. For example the Label widget will render itself onto
@@ -254,12 +252,6 @@ 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
 }