fix UITester wx - explicitly handle a KeyClick('Backspace') and update key_click_text_ctrl

[aaronayres35]

fixes #1183
fixes #1182
fixes #1177
This PR explicitly handles the case when a KeyClick command is issued with "Backspace" as the key. It also makes the same change that was added in PR #1181 for key_sequence_text_ctrl to key_click_text_ctrl
Finally, it makes the default behavior for entering a textbox to have the insertion point be the rightmost rather than the leftmost point of text. (this keeps consistent behavior with qt)

[kitchoi]

Code LGTM.
(This time let's make sure the wx + Windows build is happy with the tests on RangeEditor and TextCtrl 🤞 )

[aaronayres35]

Code LGTM.
(This time let's make sure the wx + Windows build is happy with the tests on RangeEditor and TextCtrl 🤞 )

Damn, there was one test failure on windows:

======================================================================
901FAIL: test_range_text_editor_set_with_text_valid (traitsui.tests.editors.test_range_editor.TestRangeEditor)
902----------------------------------------------------------------------
903Traceback (most recent call last):
904  File "C:\Users\appveyor\.edm\envs\traitsui-test-3.6-wx\lib\site-packages\traitsui\tests\editors\test_range_editor.py", line 83, in test_range_text_editor_set_with_text_valid
905    return self.check_set_with_text_valid(mode='text')
906  File "C:\Users\appveyor\.edm\envs\traitsui-test-3.6-wx\lib\site-packages\traitsui\tests\editors\test_range_editor.py", line 70, in check_set_with_text_valid
907    self.assertEqual(model.value, 10)
908AssertionError: 1 != 10
909
910----------------------------------------------------------------------
911

Will try to fix this now before merging

[kitchoi]

Still LGTM.

Last but not least...new flake8 errors

$ flake8 traitsui/tests/editors/test_range_editor.py 
traitsui/tests/editors/test_range_editor.py:16:1: E302 expected 2 blank lines, found 1
traitsui/tests/editors/test_range_editor.py:70:47: E225 missing whitespace around operator
traitsui/tests/editors/test_range_editor.py:73:71: W291 trailing whitespace
traitsui/tests/editors/test_range_editor.py:124:52: W291 trailing whitespace
traitsui/tests/editors/test_range_editor.py:128:1: W391 blank line at end of file