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).

src/workspace.c

@@ -392,8 +392,7 @@ void workspace_show(Con *workspace) {
  */
 void workspace_show_by_name(const char *num) {
     Con *workspace;
-    bool changed_num_workspaces;
-    workspace = workspace_get(num, &changed_num_workspaces);
+    workspace = workspace_get(num, NULL);
     _workspace_show(workspace);
 }
 
@@ -664,6 +663,22 @@ void workspace_back_and_forth(void) {
     workspace_show_by_name(previous_workspace_name);
 }
 
+/*
+ * Returns the previously focused workspace con, or NULL if unavailable.
+ *
+ */
+Con *workspace_back_and_forth_get(void) {
+    if (!previous_workspace_name) {
+        DLOG("no previous workspace name set.");
+        return NULL;
+    }
+
+    Con *workspace;
+    workspace = workspace_get(previous_workspace_name, NULL);
+
+    return workspace;
+}
+
 static bool get_urgency_flag(Con *con) {
     Con *child;
     TAILQ_FOREACH(child, &(con->nodes_head), nodes)