Available through DBus. Be aware that this does not make much
sense, as there is no way to end TabBox mode except through the
DBus interface and this does not yet select the selected window.
Therefore TabBox is changed to be controlled without pressing a
modifier key. Tab and Backtab are valid keys now which can be
used to navigate in the list and return, enter and space can be
used to close the box (and select the client).
The TabBox is exported as object /TabBox on the kwin interface
providing start and close methods and signal when the TabBox got
closed.
