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

include/data.h

@@ -141,7 +141,8 @@ typedef enum {
 typedef enum {
     FOCUS_WRAPPING_OFF = 0,
     FOCUS_WRAPPING_ON = 1,
-    FOCUS_WRAPPING_FORCE = 2
+    FOCUS_WRAPPING_FORCE = 2,
+    FOCUS_WRAPPING_WORKSPACE = 3
 } focus_wrapping_t;
 
 /**