ditch autotools and migrate to meson

Squash of the 6 main commits from Bilal
Elmoussaoui branch (no app-id changes).

Edited a bit by Arnaud Bonatti, to keep
the COPYING file, avoid adding trailing
spaces, use a variable named pkgdatadir
instead of datadir (as suggested by Jan
Tojnar), use new-lines at end of files,
consistently use 4 spaces and not tabs.

Closes !4 (see also following patches).