UI Tester update - Implement locating a nested UI for a custom style ListEditor

[aaronayres35]

Implements the seventh bullet on issue #1149 (Implement locating a nested UI for a custom style ListEditor. Consequently, one should be able to navigate into nested editors inside a nested UI.)

This PR replaces #1172

[aaronayres35]

Eventually we will be able to do this with just one KeySequence using '\b' characters, but there is a known issue with that so this works for now.

[aaronayres35]

This also gives a wx failure on windows:

======================================================================
890FAIL: test_locate_element_and_edit (traitsui.tests.editors.test_list_editor.TestCustomListEditor)
891----------------------------------------------------------------------
892Traceback (most recent call last):
893  File "C:\Users\appveyor\.edm\envs\traitsui-test-3.6-wx\lib\site-packages\traitsui\tests\editors\test_list_editor.py", line 84, in test_locate_element_and_edit
894    self.assertEqual(obj.people[7].name, "David")
895AssertionError: 'Fields' != 'David'
896- Fields
897+ David
898
899

like issue #1177
All failures appear to be coming from the same source (I suspect EmulateKeyPress in wx helpers.)

[kitchoi]

LGTM. Some minor suggestions.

[aaronayres35]

My suspicions in the above discussion were correct. I added a helper function to set the insertion point the the right not the left end of a textbox on wx. This feels a bit tangential to this PR ( I can make this change in a separate PR if needed) but without it, this PR will introduce a new test failure. After #1184 is merged, with this most recent change, there shouldn't be any new errors with this PR as far as I know.

[kitchoi]

This function combines both query and command, which often makes it more difficult to reuse and to modify. We should avoid this as much as possible. See comments in #1184

If we remove this and the changes in traitsui/testing/tester/wx/implementation/text_editor.py and the KeyClick in the test, then this PR can be reviewed and merged independently of #1184. The objective of this PR has nothing to do with the text editor anyway.

[kitchoi]

LGTM.
One function being overly indented, can be merged after fixing that.