Although theoretically GtkLabel from GTK3 should be able to replace
GeanyWrapLabel altogether, a bug [1] with it makes it use way too much
space in our about dialog (and possibly other places), making it not
really usable.

So, port the GeanyWrapLabel hack to GTK3, with the appropriate
additional hacks for it to work.  At least it looks good and don't
seem to have resizing issues now.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=657621