gtk changes in the jhbuild moduleset

[Matthias Clasen <matthias clasen gmail com>]

Hi,

we are getting ready to start development of gtk 3.90 in master.
To avoid causing lots of breakage and irritation, here is the plan:

1) Switch the modulesets to use the gtk-3-22 branch for the gtk module
2) Rename the gtk module to gtk3
3) Add a gtk4 module that follows master
4) Switch applications over to  use gtk4 as they are found to work

If you see a problem with that, please let me know.