swpanel: Consider also the release of FOCUSPREV
The switch panel was not being destroyed with the realease of
FOCUSPREV, only with FOCUSNEXT. Fix this.
One can reproduce this bug as follows.
In the "Keyboard Shortcut Preferences" of WPrefs set "Focus next window"
to e.g. "Alt+Tab" and "Focus previous window" to "Ctrl+Tab".
The switchpanel is not destroyed if we release "Ctrl+Tab" but it
is upon releasing of "Alt+Tab".
Retrieved-from: http://git.altlinux.org/people/voins/packages/?p=WindowMaker.git;a=commit;h=
51c95a55c9310f499b1fdeca138106ca7bf74423
[crmafra: Commit log]