Support localised screenshots

[aral]

Problem

If localised screenshots exist in the Flatpak metadata, they should be supported.

Currently, the first <image/> of the last <screenshot/> in the <screenshots> section is displayed.

e.g., see https://appcenter.elementary.io/org.small_tech.comet/ where the Dutch screenshot is shown on the English page.

The relevant metadata markup is:

<screenshots>
    <screenshot type="default">
      <image xml:lang="nl">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/nl/comet-basic-usage.png</image>
      <image xml:lang="tr">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/tr/comet-basic-usage.png</image>
      <image>https://raw.githubusercontent.com/small-tech/comet-screenshots/main/en/comet-basic-usage.png</image>
    </screenshot>
    <screenshot>
      <image xml:lang="nl">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/nl/comet-spell-check.png</image>
      <image xml:lang="tr">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/tr/comet-spell-check.png</image>
      <image>https://raw.githubusercontent.com/small-tech/comet-screenshots/main/en/comet-spell-check.png</image>
    </screenshot>
    <screenshot>
      <image xml:lang="nl">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/nl/comet-emoji.png</image>
      <image xml:lang="tr">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/tr/comet-emoji.png</image>
      <image>https://raw.githubusercontent.com/small-tech/comet-screenshots/main/en/comet-emoji.png</image>
    </screenshot>
    <screenshot>
      <image xml:lang="nl">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/nl/comet-settings.png</image>
      <image xml:lang="tr">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/tr/comet-settings.png</image>
      <image>https://raw.githubusercontent.com/small-tech/comet-screenshots/main/en/comet-settings.png</image>
    </screenshot>
    <screenshot>
      <image xml:lang="nl">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/nl/comet-welcome.png</image>
      <image xml:lang="tr">https://raw.githubusercontent.com/small-tech/comet-screenshots/main/tr/comet-welcome.png</image>
      <image>https://raw.githubusercontent.com/small-tech/comet-screenshots/main/en/comet-welcome.png</image>
    </screenshot>
  </screenshots>

Proposal

Initial workaround (easy)

Grab the first <image/> from the <screenshot type="default" /> in the <screenshots> section if it exists. If it doesn’t take the first image from the first screenshot (not the last), in the assumption that the author would likely have put the default first even if they didn’t mark it.

Full implementation (hard)

Actually create localised pages and serve them.

Prior Art (Optional)

The elementary OS AppCenter itself correctly displays localised pages and screenshots.

[aral]

Related: #60