update Signal to accept a slot context
replace EventTracker with EventBinder
update docs
update docs
update Widget to use EventBinders
remove the Geometry class
update Button to use new event binding
update the ZStack to take an interface instead of an Item
drop the enums file

this is not needed yet
add an interface for a layout actor
add special class to the Button when pressed

this is necessary for firefox since event.preventDefault will cause the
:active pseudo class to not be applied
fix Item css class naming
update title bar css to use button pressed class
add missing return statement
cleanup size grip
update window css for size grip changes
add move functionality to the TitleBar
update the TitleBar text cursor
wip update window
update sample page