Add appdata.xml file & improve contour.desktop file

[christianparpart]

While being in process of adding flatpak support I realized I am missing a appdata.xml file and the file name for the .desktop file isn't the best, probably.

• [x] Rename contour.desktop to org.contour.Terminal.desktop (assuming I'll keep the app ID as proposed: org.contour.Terminal).
• [x] add appdata.xml file with the same basename as above and have it installed on Linux installs into /usr/share/metainfo.
• [x] and don't forget to rename the launchable's tag in appdata.xml to the correct (new) .desktop file name.
• [ ] CI: validate desktop file
• [ ] CI: validate AppStream XML file

Additionally required checks

which are not handled by flatpak validate command but cause problems on flathub CI:

• [ ] screenshot caption must not exceed 100 chars
• [ ] There must be only a very strict subset of HTML tags in certain combinations
• [ ] <kbd> must be replaced with <code> and so should backticks.
• [ ] we could have final metadata.xml auto generated to always include all releases and their changes by parsing Changelog.md for that (and having kbd tags etc properly translated)