Refresh Appstream metadata

[karliss]

Is your feature request related to a problem? Please describe.

After #3346 it might a good idea to check if any information in appdata.xml could be update.

Describe the solution you'd like

The main candidate is various urls:

• switching main homepage URL to cutter.re from docs
• use appropriate tag for link to help/docs
• github issue tracker link
• translation contribution link (either the page in docs or link to crowdin)

Describe alternatives you've considered

Additional context

[xvitaly]

It also should be renamed from re.rizin.cutter.appdata.xml to re.rizin.cutter.metainfo.xml. I can rename it in my PR if you want.

[XVilka]

It would be nice also to have metadata for the Flathub updated. See:

• https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/
• https://flathub.org/apps/re.rizin.cutter

[karliss]

It also should be renamed from re.rizin.cutter.appdata.xml to re.rizin.cutter.metainfo.xml. I can rename it in my PR if you want.

Looks like some of the Appimage tooling doesn't fully support the new *.metainfo.xml name https://github.com/AppImage/AppImageKit/issues/1160 and still expects appdata.xml