x11: Make removal of X11 event filters safe

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)