Embed theme contents instead of bundle

[Jacalz]

Description:

This has code modernisation has various benefits. A shallow git clone of the project no longer includes all icons twice (in source and .go file), opening the huge files with bundled stuff no longer cause editors to grind to a halt but most importantly, just editing the file makes the data in the binary change without needing to do anything at all. It is all automatic and cached by the compiler. There is also less text for the compiler to parse so there might be faster compile times (assuming it can just copy the file directly into the binary) but that's just speculation.

Building fyne_demo using go build -ldflags="-s -w" yields:

• Before: 24224176
• After: 24255312

Binaries are ever so slightly larger but we are just talking ~30kb so nothing much to write home about.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.117% (+0.08%) from 59.035%
when pulling 346ab2d on Jacalz:embed-theme-contents
into 8726c8b on fyne-io:develop.

[Jacalz]

I don't know if it's just me but the workflows did seem to be done quite quickly. I suppose the double file download for that shallow git clone already is paying off? If we can do the same for the font, then I'm quite enthusiastic.

The icons are just a few kilobytes but the emoji font is 4.2mb so that might change something on slow CI runners.

[andydotxyz]

Merging to avoid a re-base due to the branch settings change