More _changed method updates to observe

[aaronayres35]

Continuation of #954, and therefor part of #899

This PR simply replaces some more _*_changed magic named static handlers with observe.

[aaronayres35]

Going to remove the WIP label on this now. This is a completely arbitrary choice, but I figure keep PRs small

[rahulporuri]

The changes themselves LGTM but I have made a bunch of suggestions - mostly related to renaming the private observe handlers. I'm okay if the suggestions are ignored but I think the suggested renaming (or any changes proposed to the suggestions themselves) are a step in the right direction for the codebase.

[aaronayres35]

CI is failing with:

[EXECUTING] /home/runner/work/pyface/pyface/.edm/envs/edm/bin/edm run -e pyface-test-3.6-wx -- python -Xfaulthandler -m coverage run -p -m unittest discover -v pyface
Shell 'sh' is not supported, things may not work properly (supported shells are bash, zsh)
pyface (unittest.loader._FailedTest) ... ERROR

======================================================================
ERROR: pyface (unittest.loader._FailedTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/__init__.py", line 61, in load_tests
    from pyface.toolkit import toolkit_object  # noqa: F401
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/toolkit.py", line 23, in <module>
    toolkit = toolkit_object = find_toolkit("pyface.toolkits")
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/base_toolkit.py", line 256, in find_toolkit
    return import_toolkit(ETSConfig.toolkit, entry_point)
  File "/home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-wx/lib/python3.6/site-packages/pyface/base_toolkit.py", line 220, in import_toolkit
    raise RuntimeError(msg)
RuntimeError: No pyface.toolkits plugin could be loaded for wx

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)
Command '['/home/runner/work/pyface/pyface/.edm/envs/edm/bin/edm', 'run', '-e', 'pyface-test-3.6-wx', '--', 'python', '-Xfaulthandler', '-m', 'coverage', 'run', '-p', '-m', 'unittest', 'discover', '-v', 'pyface']' returned non-zero exit status 1.
/usr/bin/bash /home/runner/work/_actions/GabrielBB/xvfb-action/v1/cleanup.sh
Killing the following xvfb processes: 2964
Error: The process '/usr/bin/xvfb-run' failed with exit code 1

[aaronayres35]

suspect test failure is related GH-Actions being enabled.
#950 made its way through before hand (I could have sworn I remembered gh actions being run on that PR but I must be mistaken).

[aaronayres35]

Okay CI is closer, but now I'm seeing:

======================================================================
FAIL: test_focus (pyface.tests.test_widget.TestConcreteWidget)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\pyface\pyface\.edm\envs\pyface-test-3.6-pyside2\lib\site-packages\pyface\tests\test_widget.py", line 331, in test_focus
    self.assertFalse(self.widget.has_focus())
AssertionError: True is not false

----------------------------------------------------------------------
Ran 1098 tests in 78.060s

FAILED (failures=1, skipped=29)
Command '['D:\\a\\pyface\\pyface\\.edm\\envs\\edm\\Scripts\\edm.exe', 'run', '-e', 'pyface-test-3.6-pyside2', '--', 'python', '-Xfaulthandler', '-m', 'coverage', 'run', '-p', '-m', 'unittest', 'discover', '-v', 'pyface']' returned non-zero exit status 1.
Error: The process 'C:\Enthought\edm\edm.bat' failed with exit code 1

Unfortunately I believe these are all orthogonal to this PR...

[aaronayres35]

@rahulporuri This PR still look good to go? Since your review I had to make a few CI related changes as GH Actions was enabled for the repo and CI started failing

[rahulporuri]

still LGTM