Feature: support FIXED render mode for Text

[penguinolog]

• Text was historically supported Text().pack() call
• Update SelectableIcon for FIXED support
• Force Edit to be Flow: need for cursor positioning

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 or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[coveralls]

Pull Request Test Coverage Report for Build 7169240472

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 70.359%

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Totals
Change from base Build 7168417392:	0.002%
Covered Lines:	7568
Relevant Lines:	10826

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💛 - Coveralls

[wardi]

fixed added to Edit but not Text?

[wardi]

👍 Wanted to do this for a while now, but since so many things are based on Text it might cause some unexpected issues...

[penguinolog]

Need to collect more downstream projects to test…

[penguinolog]

Tested and had no issues:
mitmproxy
socli
s-tui
nomadnet
pudb