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.

BUG: 423319


(cherry picked from commit a433fb08)