webkitgtk24: new package

webkitgtk24: new package

Add new webkitgtk24 package.

The difference with the legacy webkit package is that this one uses the
new webkit2 API, and optionally the webkit1 API.

It's versioned because the latest release of midori can't use the newer
2.6.x or 2.8.x webkitgtk versions yet.

This paves the way for a newer webkitgtk26 or webkitgtk28 package that
can live side-by-side with this one for other uses.

Some loose checks (like !xorg7) are in place for possible/future wayland
support.
Right now wayland requires gtk2, hence xorg, but it might be possible to
patch that out with some work (it seems to be done for version 2.6.x,
though the build system changed to cmake so it might not be a
straighfoward patch).

Choices had to be made in the GL-area, right now the preference order is
libGL (full OpenGL), then GLES, then nothing (obviously).
And when gtk3 is around it's also preferred as well.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>