Add support for age rating metadata

[JeffersonBledsoe]

Closes #938

[JeffersonBledsoe]

@mmatyas Am I on the right track with this PR? If so, I'll try add support for any other platforms/ providers which could provide age info (I'd have to do some research into which ones do)

[mmatyas]

The implementation in general looks good, what I'm thinking about is how this should be stored in the metadata files and how should be presented to themes.

I've just looked into the various rating systems, and while there are large variations and overlapping in the categories (eg. Australia's G/PG), it seem they can be largely reduced to a minimal age in all common cases. If we're talking about an age rating, and not purely content ratings, like Hong Kong's I/II/III, and we assume there's no upper limit on the rating, then we can reduce the field's type to an integer, as a minimal recommended age. This would also allow themes to assign some symbols (eg. PEGI icons) for the value.

Alternatively, it could be a string list, similar to eg. Kodi, where each entry is in the form Country:Rating, eg. Australia:PG. This would be probably the most precise solution, but likely an overkill, because you usually already know which region your own game is from. This would also mean more work for themes.

As for the name of the field, I think ageRating could work there too, both the API side and in the metadata file (as it's not the game's age, but its age rating we're showing there).

[mmatyas]

tl;dr: For now use int as type and call it age rating everywhere (or if you have a good idea for a name that expresses it's the minimal age, that's even better).