Add Meson build system

[jtojnar]

Resurrecting https://github.com/mypaint/libmypaint/pull/170

• [x] Checked that meson dist output looks reasonable.
• [x] Verified that build outputs the same set of files.
• [x] Checked that pkg-config files are equivalent.
• [x] Double-check that Meson build scripts match the autotools.

Fixes: #112