if the xkb-extension is enabled and the user switches between his/her
keyboardlayouts fluxbox's keybhandling doesn't work well anymore because
xkeyevent.state contains also xkb-related flags and thus we have to handle
that with caution.
KeyUtils now contain 'isolateModifierMask()' to really work only on the
modifiers. why not as part of cleanMods() ? because the XLookupString return
false results, eg TextBox's would only print chars from the first
keyboardlayout.
