Support testing with simple FileEditor (without dialog)

[kitchoi]

This PR adds testing support for the simple FileEditor via its text widget.

• DisplayedText, KeyClick, KeySequence can now be used for testing with a simple FileEditor
• A test for the FileEditor demo example is added, and an entry is added to the documentation.

This is motivated by an observation of a similar extension being made in a downstream project. One will extend using the _file_name attribute on the editor, acknowledging that such extension relies on implementation details of the editor. If the testing support is implemented in traitsui, then the support can be maintained along with those implementation details.

In the context of testing an application, it is often sufficient to set a file path via the text widget instead of via the file modal dialog.

This goes some way towards solving #1339. It also extends TraitsUI's own test coverage for the simple FileEditor.

Observations:

• The smoke tests for wx FileEditor used to fail, but not any more since Simple FileEditor cannot be launched in Wx #889 was solved.
• The entries trait on the FileEditor factory is only supported by wx. If set on wx, the simple FileEditor will have a combo box instead of a text widget. The simple FileEditor on wx is effectively two editors in one. That branch is not tested here.
• On the demo, if one sets an existing file path for the simple FileEditor, the custom FileEditor will normalize the file path. So if one sets a relative path to the simple FileEditor, they will get an absolute path back unless the custom FileEditor is removed. That's because in the demo, multiple editors are tied to the same trait.
• The test for the FileEditor demo fails on wx due to Wx Event Handler clean-up issues #752. The presence of the custom FileEditor caused it. If the custom FileEditor was removed, the test will pass for wx.

[kitchoi]

Note: One can apply more or less the same change for the simple DirectoryEditor (not covered here).

[aaronayres35]

LGTM

Just a couple comments about skipping tests on wx

[aaronayres35]

You say in the PR description: "The test for the FileEditor demo fails on wx due to #752". Should this have an @requires_toolkit(['ToolkitName.qt']) in that case?
/ a comment regarding the failure being linked to having a custom editor included in the demo?

[kitchoi]

Good point. Yes I should add the decorator.

[kitchoi]

Yes I should add the decorator.

On second thought, this is a demo test so I'd want to avoid importing anything from traitsui.tests._tools.
Will go with a comment instead :/