fixes #776

this implements both the "move container to workspace back_and_forth" command
and movements to the same workspace when auto_back_and_forth is set.

it includes documentation and test suite additions by michael.

it also simplifies the workspace_show_by_name function (making use of
workspace_get accepting NULL pointers).

parser-specs/commands.spec

@@ -243,6 +243,8 @@ state MOVE_WORKSPACE:
       -> MOVE_WORKSPACE_TO_OUTPUT
   workspace = 'next', 'prev', 'next_on_output', 'prev_on_output', 'current'
       -> call cmd_move_con_to_workspace($workspace)
+  'back_and_forth'
+      -> call cmd_move_con_to_workspace_back_and_forth()
   'number'
       -> MOVE_WORKSPACE_NUMBER
   workspace = string