Export GES library API in headers when we're building the
library itself, otherwise import the API from the headers.
This fixes linker warnings on Windows when building with MSVC.
Fix up some missing config.h includes when building the lib which
is needed to get the export api define from config.h
Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/issues/42
We were only concidering that we should let the group handle moving
transitions when changing transitions but in fact as soon as a
transition is happenning between two clips that are in a same group
the group properly handles moving the transition, so let the
group do its job.
Fixes T7543
Differential Revision: https://phabricator.freedesktop.org/D1281
Fix all tests as we now have 1 priority inside the layer
dedicated to transitions (basically no source clip will
ever have a priority of 0 inside a layer).
Differential Revision: https://phabricator.freedesktop.org/D1276