Implement focus_wrapping workspace

I had a dilemma about the behaviour here:
1. Prohibit focus leaving the workspace in any case unless if
get_tree_next's initial argument is a workspace. This is what this
commit does (also i3-cycle).
2. Leave the workspace if no warp is possible (eg workspace with single
container or `focus right` with `V[a b c*]`).

Fixes #2180

src/config_directives.c

@@ -268,6 +268,8 @@ CFGFUN(disable_randr15, const char *value) {
 CFGFUN(focus_wrapping, const char *value) {
     if (strcmp(value, "force") == 0) {
         config.focus_wrapping = FOCUS_WRAPPING_FORCE;
+    } else if (strcmp(value, "workspace") == 0) {
+        config.focus_wrapping = FOCUS_WRAPPING_WORKSPACE;
     } else if (eval_boolstr(value)) {
         config.focus_wrapping = FOCUS_WRAPPING_ON;
     } else {