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

parser-specs/config.spec

@@ -215,7 +215,7 @@ state MOUSE_WARPING:
 
 # focus_wrapping
 state FOCUS_WRAPPING:
-  value = '1', 'yes', 'true', 'on', 'enable', 'active', '0', 'no', 'false', 'off', 'disable', 'inactive', 'force'
+  value = '1', 'yes', 'true', 'on', 'enable', 'active', '0', 'no', 'false', 'off', 'disable', 'inactive', 'force', 'workspace'
       -> call cfg_focus_wrapping($value)
 
 # force_focus_wrapping