WIP: Add a script to set a nightly version release at build time

Closes #1152.

@mjog I gave it a try. What do you think? Too hackish?

The main version in the script (now 40) should be updated after a new stable release. I don't think I can have access to meson.project_version() from the shell script, right?

Also, I'm setting the MR as WIP, because the meson run target (called nightly) is not run yet. My idea is adding a meson compile nightly command to the geary module in the flatpak .yaml manifest. I guess I should turn the buildsystem from meson to simple to add above command?