wms/s: Special case "MainWindow" instead of "buddylist" window

This patch changes the sample window manager from demonstrating
specially handling a window named "buddylist" to handling one named
"MainWindow" instead. This is an additional change in order to make
the sample window manager more generic and less bound to its
libpurple roots. The keyboard shortcut was kept as Alt+B as I
couldn't find a more appropriate alternative which wasn't already
used.