Refined the fullscreen focus constraints.

Basically, a focus change can't escape a fullscreen container. The
only exception is per-output fullscreen containers, as you should
be able to focus a container in a different workspace in this case.

This is an improvement on 4eab046e, now considering the difference
between global and per-output fullscreen and taking the tree
structure into account to determine what escaping the fullscreen
container means. It only affects targeted focus commands in the
form "for_window [...] focus", but it lays the foundation for
forthcoming fixes to all other focus commands.

src/commands.c

@@ -1262,13 +1262,9 @@ void cmd_focus(I3_CMD) {
         if (!ws)
             continue;
 
-        /* Don't allow the focus switch if the focused and current
-         * containers are in the same workspace. */
-        if (focused &&
-            focused->type != CT_WORKSPACE &&
-            focused->fullscreen_mode != CF_NONE &&
-            con_get_workspace(focused) == ws) {
-            LOG("Cannot change focus while in fullscreen mode (same workspace).\n");
+        /* Check the fullscreen focus constraints. */
+        if (!con_fullscreen_permits_focusing(current->con)) {
+            LOG("Cannot change focus while in fullscreen mode (fullscreen rules).\n");
             ysuccess(false);
             return;
         }