Clean up some things, fix screenshots model
carousel: Moving code around, adding import aliases
ScreenshotsModel: Export Roles enum
It's better to have a grip on the whole set of roles identifiers instead of having a single unconventional getter. QML/JS may query the model for any role with data() method.
ScreenshotsModel: Fix code style
AppStream: Fix swapped thumbnail and full image URLs
Also, what's the deal with that condition?
thumbnail.isEmpty() ? full : thumbnail
I'd prefer client code handling the absence of a proper thumbnail, instead of making assumptions and ending up loading full image twice.