Start of an extendified notification applet.
Not only does this version use extenders for displaying notifications, it's also based on PopupApplet which means removal of a lot of duplicate code.

Code can maybe be cleaned up a bit more, but it basically works.

Only major regression: it no longer groups notifications of the same app.
To do this neatly, it would require grouping support in extenders and that's more something for 4.3 I think.