Minor improvements regarding files' install locations

[a-wai]

Hi,

While working on packaging passes for Debian, I came across a few "issues" with the locations some files (appstream metadata and libbarcode-content-encoder.so private library) were being installed to.

This PR:

• renames the appstream metadata file and installs it to the proper location
• installs the private library to a subfolder of the system's libdir

[pablo-s]

Thanks for your contribution!