Part of https://gitlab.gnome.org/GNOME/Initiatives/issues/1;
Sets up dual installation for flatpaks, allowing us to
build and run geary without affecting our system install.
This will allow testing of nightlies and CI artifacts as well
via the flatpak, so we can iterate on designs and share
bundles to test with.
Some remarks:
* Note that Meson adds a hard dependency on Python 3.
* All dependencies and defines are now listed together.
* Some build files were put in their respective subdirectories, e.g. the Geary
engine library will be built from the Meson file in `src/engine`.
* `--fatal-warnings` is no longer an explicit flag, as Meson provides
`-Dwerror=true` for this.
* An explicit resource file needs to be used. The issue to support this from
Meson itself can be found at https://github.com/mesonbuild/meson/issues/706 .
* The `gnome.yelp()` function parses a LINGUAS file so we no longer need to keep
track of all languages in our build system.
* There are no Debian scripts defined in the meson.build files to keep them
clean, but they can be kept as separate scripts in `build-aux`.
* Left out the `dist` target as there is now `ninja dist`
* `geary-docs` is disabled by default, as valadoc-0.38.3 returns errors.
https://bugzilla.gnome.org/show_bug.cgi?id=777044
