Allow focus child/parent when in fullscreen.

This is now restricted according to the already defined fullscreen
focus constraints. Test case 157 was removed, as we don't prevent
level up/down in fullscreen anymore. Those commands are properly
tested in fullscreen by test case 156.

Fixes: #612

src/con.c

@@ -1170,6 +1170,10 @@ bool con_fullscreen_permits_focusing(Con *con) {
     if (fs->type == CT_WORKSPACE)
         return true;
 
+    /* Allow it if the container itself is the fullscreen container. */
+    if (con == fs)
+        return true;
+
     /* If fullscreen is per-output, the focus being in a different workspace is
      * sufficient to guarantee that change won't leave fullscreen in bad shape. */
     if (fs->fullscreen_mode == CF_OUTPUT &&