This patch removes the GTK2 code and fully uses GTK3.
The minimum required version is 3.4, which is present on Debian Stable.
On MS Windows, version 3.6 is available.
There are only a few (cosmetic) issues left, but I think the transition to GTK3 can now be made, and it will not be worth the effort to maintain backward compatibility with GTK2.
And here is the patch...
Applied to svn 2048 (GTK3 port) and 2049 (MinGW update) for Pioneers 15.2