wms/s: Remove libpurple dependency from sample wms

As far as I could tell, wms/s.c is a sample window manager, made
to demonstrate how to use various features of libgnt. Therefore,
it's a little silly to depend on libpurple functionality for it.
This patch removes the dependency of libpurple from the sample wms
by changing it from trying to toggle the buddy list visibility upon
pressing the keyboard shortcut, to simply presenting any window
named "buddylist".