#16 Install metainfo files in 'metainfo' directory (not 'appdata')

[Flow]

The /usr/share/appdata location is deprecated, it should not be used
anymore by new software. Appdata/Metainfo files should be installed
into /usr/share/metainfo directory.

For more details, please see the freedesktop Upstream Metadata
guidelines at
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html

[Felipe Castro]

Done since 3.14