GPA conversion to GTK3

[Andreas Rönnquist]

Hi!

I have been hacking on Gnu Privacy Assistant (GPA), porting it to GTK3,
and I have come quite far - If you have any interest in it, please
check it out on https://github.com/gusnan/gpa (port-gtk3 branch).

I have converted the menus to the modern GtkBuilder style, and I have
moved the xpm images to the source folder (It might not have been
necessary, I'm not sure), but it works just fine now. The GTK3 style is
to compile images to C source, and including it in the program
compilation (Which avoids the necessity to load the images from disc
runtime). 

There's still warnings about alignment stuff (I'm not sure on how to
handle those), and there's a g_cclosure_marshal_* that I am not sure I
have handled correctly - needs investigation.

I might have missed connecting menu items and signals properly, so
please try it out and report back here (or as github issues / pull
requests).

Please notice that this is nowhere near release quality yet.

And yes, I have noted that gpg4win has abandoned gpa, which I guess is
part of the reason of the lower priority for it, but that doesn't mean
that us Linux people cannot use it, right?

Another thing that probably should be mentioned is that I use
GtkToolbar to make it work as close to the GTK2 version as possible,
but for GTK4 this widget is removed, and needs to be replaced with
something else in the future. (Let's hope that is some time away).

And I don't mind horribly if the code isn't used, it was a nice exercise.

best
/Andreas Rönnquist
gusnan at librem.one