age-rating-context-dialog: Have a single row per group

The order of importance is: important (red), warning (yellow), neutral (grey), unimportant (green, slashed icon). So neutral attributes now come before unimportant ones. Attributes whose rating is "unknown" now have a proper description shaped like "No information regarding …", based on the phrasing of the "unimportant" ratings in AppStream.

We now have a single row per age rating group (6 groups total), we use the color and icon for the strongest importance in that group, we append the descriptions (separating them with " • ") and we sort the descriptions by importance. If there are only unimportant attributes, we show a predefined "nothing to see here" description for the group, otherwise we skip the unimportant ones to focus on the ones that matter to the users.

If there is a single rating in the group, we still use its dedicated icon, title and description.

Fixes #1345 (closed)

CC @bertob

Example 1

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Example 2

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Example 3

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