Add meson subprojects

Meson provides a way to use wrap subprojects as a fallback in the dependency command. Many GNOME projects use subprojects already, and because I don't want to manually build libdazzle 3.37 every time I compile Builder without Flatpak I added some fallbacks for GNOME projects that use Meson.

Please note:

  • I had to move libglib_dep up as it redefines some commands when it's used as a subproject.
  • The libraries are fetched from git master versions. Distributing a stable release with this would lead to "unstable" master versions being built in stable releases when meson falls back. However, setting revision to a tag would require updating the wrap files every so often. I don't know what's better.
  • I couldn't set up a wrap file for vte as it requires cpp_std=c++17 and fails to configure with Builder's version.

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