Add an "Above_Tab" pseudo-keysym

We want switching between the windows of an application to be an easily
accessible operation. The convenient and memorable keybinding is the
key above the tab key - but the keysym for that key isn't consistent
across different keyboard layouts.

Add code that figures out the key from the XKB geometry and a magic
keysym name "Above_Tab" that refers to this key and switch
the default binding for cycle_group to <Alt>Above_Tab. (This will
have no effect for the normal case of getting the key binding from
GConf until this patch is applied to Metacity as well.)

https://bugzilla.gnome.org/show_bug.cgi?id=635569