To avoid compiler warning when using const string to create a new GESUriClip as string is not modified and only passed to functions which take a const string. https://bugzilla.gnome.org/show_bug.cgi?id=766523
And GESCustomTimelineSource to GESCustomSourceClip
Conflicts: ges/ges-pitivi-formatter.c ges/ges-uri-clip.c tests/check/ges/project.c tests/check/ges/uriclip.c