Make number of workspaces dynamic (no longer limited by 10)

Warning: This is not yet thoroughly tested, so be prepared to
encounter some segfaults. Please enable logging and coredumps,
so we can fix bugs quickly.

src/util.c

@@ -455,7 +455,7 @@ Client *get_matching_client(xcb_connection_t *conn, const char *window_classtitl
         }
 
         LOG("Getting clients for class \"%s\" / title \"%s\"\n", to_class, to_title);
-        for (int workspace = 0; workspace < 10; workspace++) {
+        for (int workspace = 0; workspace < num_workspaces; workspace++) {
                 if (workspaces[workspace].screen == NULL)
                         continue;