Asset repo size

[RobLoach]

@RogueScholar mentioned we can run into build limits when getting the assets repo.

[baxysquare]

There's multiple ways we can work toward reducing the size of the repository.

I'm willing to empty the directories for Systematic, FlatUX and Automatic and leave a ReadMe that encourages people who are interested to download the source files at baxy-retroarch-themes. As far as I can tell, that would result in an immediate reduction in size.

I'm also willing to remove all the directories that were put in place in preparation for universal themes. I believe these include cfg, devtools, fonts, and sounds. These directories were intended to pool common assets that any theme could make a call to them, thus saving more space. But it's never been implemented, so it's currently taking up unnecessary space.

To my knowledge, the directories that are absolutely required are branding, glui, menu_widgets, nxgui, ozone, pkg, rgui, and xmb. I don't know if there are any dependencies in src or any other directories in the repository. Is there any way to confirm that?

Long-term, I think it would be wise to have a requirement for all themes to have a set of SVG source files, so we could dump all the PNGs from the repository, and have them created on-demand as part of the build process. This would allow for scalability based on the needs of each build. But that would mean we'd have to retire some themes that only have PNG source. That might not be necessary now, but should be considered in the future.

I'm open to suggestions and feedback. Please advise on how you think it is best to move forward.