appdata: Improve appdata for AppStream 1.0

appdata: Improve appdata for AppStream 1.0

• Add the <developer><name> tag
• Use appstreamcli to validate appdata
• Add the vcs-browser and the translate URLs
• Update the homepage URL
• Add missing the launchable tag

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This picks !56 (closed) were it left off, but adding a comment above <developer_name>. This MR passes

flatpak run --command=flatpak-builder-lint org.flatpak.Builder appstream org.gnome.font-viewer.appdata.xml.in