tkabber: Merge main package with site plugins.

The site plugins are released alongside the main Tkabber sources, so it
makes no sense to have them in a separate package (which also introduces
an impurity). In addition, both packages share the same makefile
structure, so it really makes sense to merge them.

Before people might get worried about my decision to enable those
plugins by default: Since version 1.0, Tkabber is no longer loading
_all_ available plugins, but gives you a menu (Plugins Management) to
selectively enable plugins (whereas all plugins are disabled by
default).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>