Metainfo: Rename and improve

[ryonakano]

Inspired by similar PR in elementary Music.

• Rename to .metainfo.xml.in which recommended by fd.o
• Use "desktop-application" instead of deprecated "desktop" for component tag
• Use fixed branch in screenshot URL
• Use fd.o standardized brand tag instead of custom tag by elementary
• Add supports tag

References:

• https://freedesktop.org/software/appstream/docs/sect-Metadata-Application.html
• https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html