[GPA] Port to GTK3

[Andreas Rönnquist]

Hi

I have managed to build GPA against GTK3 - is there any interest at all,
or is GPA completely dead? It builds and runs fine in my minor testing.

I am doing this because Debian is on the mission to remove GTK 2 sooner
or later - it might take quite some time, but there have already been
bugs reported to remind us Debian maintainers. (see the bug on gpa here
[1])

I have made my fork available on github in the port-gtk3 branch [2],
feel free to import it to the GnuPG git repositories. If you want the
commits/changes in another way, please let me know.

There are some places where I have removed some deprecated calls that
doesn't have a simple replacement, which might change the experience
some - see [3].

Also, I am not sure about the best way to handle the gtk_marshal stuff [4],
some extra eyes would be greatly appreciated.

As is overall testing, to see if behaviour has changed in unexpected ways.

So far I have made it build, it needs way more work to remove all stuff
that gets warnings during the build for use of deprecated functions.

best
/Andreas Rönnquist
gusnan at librem.one

1: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=967447
2: https://github.com/gusnan/gpa
3: https://github.com/gusnan/gpa/commit/4031700ffd81613f8b985e6af8a80192d858364e
4: https://github.com/gusnan/gpa/commit/95e07080a2a08196cafb05b69345ea1d629424b1