switch to meson and uses RDNN

This PR shouldn't be merged yet.

This PR:

• Removes autotools completly
• Switch to Meson instead
• RDNN icon names and other stuff.
• Moves the libgd from the source directory to subprojects subdir (required by meson subproject).
• Removed README as it was pointing to README.md anyway.
• Renamed Authors.in to Authors.md (it's a markdown after all) the naming was forced by autotools.
• Renamed Install to Install.md (the same reason as Authors.in)
• Renamed News to News.md (the same reason as above)
• Added meson and ninja-build to .gitlab-ci.yml (the deploy and other sections must be updated).

Questions:

• Not sure where I should install libgd? So I set the pkgdatadir to be the same one as the one used by gnome-music itself. The same for pkglibdir, with a lib suffix. Should be fixed?
• Not sure about the required stuff form libgd, so I enabled everything. Must be verified before merging. (Line 31 in meson.build)
• I have switched the gettext domain name to use the RDNN too. Not sure if other apps do that too? Should I rename it back? The reason why I did that, is to keep things consistent with the rest of the namings. And so I can use meson.project_name() instead of hardcoding stuff.
• The default project name is now org.gnome.Music and so, the datadir is $PREFIX/share/org.gnome.Music. Gnome Characters does the same thing. Not sure about the rest, as I'm using mostly flatpak apps...
• I have added two tests cases on the meson build system to validate the desktop file and the appdata file. Should those be added to the to the gitlab-ci tests?

This is my very first contribution to gnome-music! Hope it's quite good 💯