Port gst-project-maker to meson

Hi,

here is the port to meson of gst-project-maker.

There are still some differences with the autotools version:

1. Do not add gstreamer-controller-1.0 to the default dependencies.
2. The -Wall option is not set explicitly.
3. The installation path is not adjusted to "$HOME/.gstreamer-1.0/plugins" when installing into $HOME.
4. The flags in GST_PLUGIN_LDFLAGS have not been ported to meson.

I am not sure about 1.

Meson should handle 2., right?

About 3. I don't know how to get env vars from within meson.

I'd like some advice about 4 too, I don't see those flags used in meson.build files anymore, are they still needed fo external projects?

Thank you, Antonio