Added the portable code for Area.
Added the GTK+ implementation of Area. It's not enabled yet due to cgo bugs, and there's some extra work needed (started here) to tie this in...
Added Area support in sysdata.go so the package can compile again. Now to do that big change mentioned in the previous commit: overhaul of the GTK+ signal system.
Major overhaul of the way GTK+ events are handled. Closures that perform events are no longer generated; instead all events are given to fixed, compile-time, cgo-exported callback functions that take the sysData as user data. This saves memory (we no longer need to generate closures and we no longer have to save them elsewhere to keep them from being garbage collected) and makes the top of sysdata_unix.go slightly cleaner, but it moves the code for handling UI events into callbacks_unix.go. This is needed for Area, in which we need to feed the sysData to an event connected to a subwidget instead of the main widget returned.