Flatpak module: Add specific variables to certain packages

[dominichayesferen]

Of course, these would be specified in the curated package data.

For example: Apply Adwaita themelock if adwaita-gtkthemelock is True in the ID's metadata, or allow home access if allow-home is True in the ID's metadata, or set the XDG_CURRENT_DESKTOP to Pantheon if force-pantheon is True in the ID's metadata

We'd also want to do this conditionally based on source (or 'all' for source-independent), so that for example force-pantheon can only be triggered for Epiphany from AppCenter, etc.

Note: Would also need to figure out undoing this once no longer True