libgtk3: new package

This package provides Gtk+ 3.0, a graphical toolkit.

[Peter: fix Config.in deps, ensure atleast 1 backend, remove redundant info]
Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
[Eric: added backend support, use of pkgconf for host-libgtk3]
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>