Meld nightly flatpak builds

[Kai <kai willadsen gmail com>]

Hi all,

Meld now has "nightly" flatpak builds from CI, published as
`org.gnome.MeldDevel` to the GNOME nightly repo (see
https://wiki.gnome.org/Apps/Nightly).

Somewhat related to this, I've updated Meld's build instructions for
the now-supported Meson build system. We still have the old
setuptools/distutils-based build in place, but it's now *only* used
for Windows builds.

I'm extremely confident that there will be issues with both of these
changes, so please do file bugs.

It's worth mentioning that getting packages from your
distribution/Flathub/Windows binaries are still the suggested way to
run Meld for most people... but I'm hopeful that the nightly flatpak
option will help get some more testing on the development branch.

cheers,
Kai