Extend UITester support for TitleEditor

The goal would be to extend support for downstream projects to write tests that require interacting with the GUI objects created by TitleEditor.

Actions:
(1) Add (toolkit agnostic) tests for the editor in TraitsUI, focus on interactions that are going to be useful downstream as well
(2) Expand the testing support via the default target registry used by UITester

Optional and opportunistic: See if a test can be added for the demo using this editor (if there is one), related to #1257. It might also provide motivation as to what features are likely useful downstream.