Feature: Add focus reporting support

[aclindsa]

This allows an application to request that focus events (i.e. when focus is gained or lost by the application) be reported as keypress events "focus in" and "focus out". It is currently only supported by raw POSIX displays.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment (note: some stuff in my environment was funky so I had to manually re-run some of the tests tox was running on the command-line, but I believe I was able to get everything to pass that way)
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

(P. S. If this pull requests fixes an existing issue, please specify which one.)

[coveralls]

Pull Request Test Coverage Report for Build 10929480676

Details

• 5 of 7 (71.43%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 74.748%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
urwid/display/_posix_raw_display.py	3	5	60.0%

Totals
Change from base Build 10878763339:	0.002%
Covered Lines:	9418
Relevant Lines:	12748

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💛 - Coveralls

[aclindsa]

@penguinolog Thanks for reviewing/merging this! Do you know when the next planned release is? (not urgent)

[penguinolog]

I hope to see PR for Chinese Unicode input under windows (bug and code sample is published) and then make release.

[aclindsa]

I hope to see PR for Chinese Unicode input under windows (bug and code sample is published) and then make release.

Awesome, thanks for all your work maintaining urwid!