gst-full improvements to support external subprojects.

This MR aims to improve the use of gstreamer full by external subprojects.

A test has been renamed to check libgstreamer-full.so.

commit 42c48200 (HEAD -> dab_gst_full_improvments, dabrain34-gitlab/dab_gst_full_improvments) Author: Stéphane Cerveau scerveau@collabora.com Date: Thu Jul 16 18:13:50 2020 +0200

gstreamer-full: add unit test

This test allows to test gstreamer-full behaviour

commit 5c8fd422 (HEAD -> dab_gst_full_improvments, dabrain34-gitlab/dab_gst_full_improvments) Author: Xavier Claessens xavier.claessens@collabora.com Date: Sun Jul 26 08:00:58 2020 -0400

gstfull: Also include 'sources' from dependencies

It contains generated headers like mkenums.

commit 88a72d12 Author: Xavier Claessens xavier.claessens@collabora.com Date: Fri Jul 24 11:15:44 2020 -0400

gst-full: Add missing include directories to use it in subprojects

commit 5aabaaac Author: Stéphane Cerveau scerveau@collabora.com Date: Wed Jul 15 16:57:47 2020 +0200

gst-full: declare dependency

Declare the gstreamer-full dependency which can be used
later by 3rd party applications.

commit 369031ab Author: Stéphane Cerveau scerveau@collabora.com Date: Wed Jul 15 15:46:49 2020 +0200

meson: move custom_subproject definition for gst-full

In order to let any subproject to use gstreamer-full
declare the subprojects after gstreamer-full definition.