Ensure TextInfo.getTextInChunks does not freeze, and provide new friendly compareable TextInfo endpoint properties

[michaelDCurran]

Link to issue number:

Fixes #11613

Summary of the issue:

TextInfo.getTextInChunks could sometimes end up in an infinit loop when dealing with particular TextInfo implementations such as ITextDocument where setting start to the end of the document results in the start never quite getting there.

Description of how this pull request fixes the issue:

To work around this specific freeze, getTextInChunks now double checks that the start has really moved to the end of the last chunk. If not, we break out of the loop.
However, for a long time now we have wanted to make these TextInfo comparsons much more readable, which can aide in finding logic errors much easier.
To that end, TextInfo objects now have start and end properties, which can be compared mathematically, and also set from another, removing the need to use compareEndPoints or setEndPoint.
Some examples:
Setting a's end to b's start
Original code:

a.setEndPoint(b, "endToStart")

New code:

a.end = b.start

Is a's start at or past b's end?
Original code:

a.compareEndPoints(b, "startToEnd") >= 0

New code:

a.start >= b.end

TextInfo.getTextInChunks has been rewritten to use these new properties.

Testing strategy:

• Performed exact steps in NVDA crashes when collecting text in Chunks from its edit controls #11613. No freeze occurs now, and all lines are correctly returned.
• Performed the same steps but with a simple document in notepad. Previously this did not freeze, and it still does not. All lines are correctly returned.
• Added unit tests for comparisons of start and end properties for less than, less than or equal to, equals, not equals, greater than or equal to, and greater than.
• added unit tests for setting start and end.

Known issues with pull request:

None.

Change log entry:

Changes for developers:

• TextInfo.getTextInChunks no longer freezes when called on Rich Edit controls such as the NVDA log viewer.
• TextInfs now have start and end properties shich can be compared mathematically with operators such as < <= == != >= > and also can be set from each other. Thus it is now preferred to write code such as a.start <= b.end rather than a.compareEndPoints(b, "startToEnd") <= 0. And a.start = b.end rather than a.setEndPoint(b, "startToend").

Code Review Checklist:

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• Pull Request description is up to date.
• Unit tests.
• System (end to end) tests.
• Manual tests.
• User Documentation.
• Change log entry.
• Context sensitive help for GUI changes.

[AppVeyorBot]

• PR introduces Flake8 errors
• Build (for testing PR)

See test results for failed build of commit 541d42907d

[feerrenrut]

This looks great, way more readable.

[feerrenrut]

Looks good to me!

[cary-rowen]

This change should belong to "Changes for developers" but it appears in "Bug Fixes".