Implement 'all' matching criterion

This criterion matches all open windows, as a more readable (and
correct) version of the 'class=".*"' criterion (more correct because
it'll also match windows which don't have WM_CLASS set yet).

include/data.h

@@ -530,6 +530,7 @@ struct Match {
            WM_FLOATING_USER,
            WM_FLOATING } window_mode;
     Con *con_id;
+    bool match_all_windows;
 
     /* Where the window looking for a match should be inserted:
      *