Improve metainfo

[SeaDve]

• Add brand color, like the following:

<branding>
  <color type="primary" scheme_preference="light">#ff00ff</color>
  <color type="primary" scheme_preference="dark">#993d3d</color>
</branding>

• Add screenshot captions, like the following:

<screenshots>
  <screenshot type="default">
    <image>https://example.org/example1.png</image>
    <caption>A caption</caption>
  </screenshot>
  <screenshot>
    <image>https://example.org/example2.png</image>
    <caption>A caption</caption>
  </screenshot>
</screenshots>

• Use proper screenshot image tag

The link should be based of a tag or commit instead of branch.