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/commands.h

@@ -55,6 +55,12 @@ void cmd_criteria_add(I3_CMD, char *ctype, char *cvalue);
  */
 void cmd_move_con_to_workspace(I3_CMD, char *which);
 
+/**
+ * Implementation of 'move [window|container] [to] workspace back_and_forth'.
+ *
+ */
+void cmd_move_con_to_workspace_back_and_forth(I3_CMD);
+
 /**
  * Implementation of 'move [window|container] [to] workspace <name>'.
  *