New properties are added to EventData for Supervisor events:
* Widget: a reference to the widget which is receiving the event.
* Clicked (bool): for MouseMove events records if the primary button is pressed.
* func RelativePoint(): returns a version of EventData.Point adjusted to be
relative to the Widget (0,0 at the Widget's absolute position on screen).
Other changes:
* Destroy() method for the Widget interface: widgets that need to free up resources
on teardown should define this, the BaseWidget provides a no-op implementation.
* Window.Resize() will properly resize a Window.
* Window.Center(w, h int) to easily center a window on screen.
* Added the TabFrame widget with an example program and screenshot
* Button: FixedColor=true to set a consistent background color and not
worry about it with mouseover/down events.
* New and completed widgets: Menu, MenuButton and MenuBar.
* MenuButton is a kind of Button that opens a popup Menu when clicked.
* MenuBar is a container of buttons designed to be attached to the top
of an application window ("File, Edit, View, Help")
* Supervisor manages the popup menus with its new concept of a Modal
Widget. Modal widgets take exclusive event priority for all mouse and
key events. The pop-up menu is a modal window, which means you must
click an option inside the menu OR clicking outside the menu will
close it and eat your click event (widgets outside the modal don't
receive events, but the modal itself gets an event that you've done
this).
