The panel does not depend anymore on libglade: all the UI has been moved to GtkBuilder and glade-3.6.
All the additional applets, previously hardcoded as custom widgets in libglade, have been rewrote practically from scratch as self-contained widgets. To be able to access them from glade-3, the code has been moved in its own shared library, libgpwidgets. This library can be seen and used outside gtk2panel; it is basically a collection of GtkWidgets.
Glade now supported: this means the panel can be designed by adding and configuring widgets using glade-3.
The build system changed deeply to not hardcode any dependency: only Gtk+2 is required. libwnck related widgets can disabled from configure (and the glade support too).