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).

include/workspace.h

@@ -95,6 +95,12 @@ Con* workspace_prev_on_output(void);
  */
 void workspace_back_and_forth(void);
 
+/**
+ * Returns the previously focused workspace con, or NULL if unavailable.
+ *
+ */
+Con *workspace_back_and_forth_get(void);
+
 
 #if 0
 /**