Port to meson (linuxmint#258)

* Port to meson

* plugin generation script: swtich to meson, update to python3, switch to libpeas, add some extra options, and cleanup

* clean up some build warnings

* kill xed-bugreport.sh with fire: it isn't used anymore, and probably doesn't even work

* update gzip command to avoid warnings on some systems and move appdata.xml to /usr/share/metainfo/ as that's where it's supposed to go now

* POTFILES.in: fix path that changed in the meson port, which was causing makepot to fail