Preserve back_and_forth during restart

Add new key "previous_workspace_name" to the json dump of the root container.

testcases/t/176-workspace-baf.t

@@ -163,6 +163,14 @@ cmd 'scratchpad show';
 cmd 'workspace back_and_forth';
 is(focused_ws, '6: baz', 'workspace 6 now focused');
 
+################################################################################
+# See if BAF is preserved after restart
+################################################################################
+
+cmd 'restart';
+cmd 'workspace back_and_forth';
+is(focused_ws, '5: foo', 'workspace 5 focused after restart');
+
 exit_gracefully($pid);
 
 done_testing;