Fix crash in Workspace::workspaceEvent()
If an X11 event filter has been activated and it unregisters another X11 event filter, then the window manager may crash because the foreach macro in Workspace::workspaceEvent() makes a copy of m_genericEventFilters or m_eventFilters and we can call the event() method for an already defunct filter.
With this change, X11 event filters can be safely removed and installed at any particular moment.