[DefKorns]

First steps of restructuring om_vars in place, one menu at a time. The file and themeGenerator keep their names, since the published catalog's get_<theme> scripts source om_vars and call themeGenerator.

• Downloads: the catalog functions grouped in a #### DOWNLOADS section; rm -f ./*.tar.gz instead of a shellcheck disable; markNewThemes uses locals.
• Installed themes: opening the Theme Manager after applying a theme or installing one no longer rebuilds all 85 commands (~1.6s + loading screen). themeSync adds the missing themes, removes the gone ones and rewrites the delete line of the previous and the new applied theme (~0.09s). The full themeLoader stays for reload and first run.
• Folder Themes: the themes list is built with one find for the music check and shell reads for title and preview (~1.1s to ~0.5s, output identical).

Tested on a SNES console (SFC firmware, 85 themes): incremental output compared file by file with the full rebuild.

Stacked on #18.