Feature: support FIXED render type for GridFLow

[penguinolog]

• GridFlow knows the amount and size of contents + size of spacing
• Move GridFlow tests from test_container to test_grid_flow

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)

[wardi]

the fixed widget color is on the right in all the others, better to be consistent

[penguinolog]

I copy-pasted background from urwid_widgets_1

[wardi]

interesting. I wonder why I flipped the order between urwid_widgets_1 and urwid_widgets_2 🤔

Still, better to be consistent within the same diagram at least.

[penguinolog]

Done

[coveralls]

Pull Request Test Coverage Report for Build 7490198737

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 69.787%

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Totals
Change from base Build 7489926545:	0.04%
Covered Lines:	8251
Relevant Lines:	11912

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💛 - Coveralls

[wardi]

other than the image color issue this looks good