Extend UITester support for TableEditor

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

For the Qt part of the implementation, there are existing functions that interact with an QAbstractItemView (see https://github.com/enthought/traitsui/blob/94041ba0697cddc09fec5bcef5b462e36f4c1b55/traitsui/testing/tester/_ui_tester_registry/qt4/_interaction_helpers.py#L177), which may be reused.

The implementations for this editor are likely reusable with the implementations for TabularEditor as well.

Actions:
(1) Look into existing tests for the editor and decide if any of the interactions are going to be useful downstream.
(2) Add more tests for the editor with any new interactions that are going to be useful in downstream code as well.
(3) Expand the testing support via the default target registry used by UITester.
(4) Convert the existing tests for the editor to use 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.