feat: import non-game extras from GOG and Humble

[Rast1234]

Optional feature for GOG and Humble plugins to import non-game extra content like soundtracks, artbooks, ...

• Implemented as additional checkbox in both plugins settings
• Imported items have extras category
• They are just direct downloads, not something "installable" and "launchable". Every item has a link for download. Install button opens browser with that link.
• Not sure if i should update all localization resources? Please advice
• GOG import takes more time because there's a HTTP request for every game

if this gets merged, i'd like to do something for Steam too

build of both plugins to try: build.zip