Add about dialog and window for helping apps make a pretty about screen

[andydotxyz]

Matching the parallax look in develop version of fyne_demo.
Looks sweet :)

also added a new general “fyne-x” demo command for when we don’t really need a full app to show off a new component.

[coveralls]

Pull Request Test Coverage Report for Build 8433207695

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 47.817%

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Totals
Change from base Build 7933141749:	0.0%
Covered Lines:	1884
Relevant Lines:	3940

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💛 - Coveralls

[Jacalz]

Nice stuff. Kind of related to Jacalz/rymdport#143 which I opened recently.

Something about having the version above seems strange to me. Maybe we can consider something like the following screenshot (smaller text below the icon)?

[andydotxyz]

That's all addressed now (and new images added to show)

[Jacalz]

Thanks. This is looking perfect now. I haven't been able to test it locally (due to being away from my laptop) but the code looks great.