GTK3 is a no-go. If you can build a usable IDE with it, it's not ready.
Yes, while it has made heaps of progress recently, still a bit off. And even if we concentrated on getting a perfect IDE, how credible would it be unless tested widely against "out in the wild" apps ?
In Debian, my app, tomboy-ng, is built using Qt5. And it works very well indeed. But a Debian release comes with an appropriate libqt5pas, people who just send out compiled binaries to whoever will need to package ! And some end users will need to upgrade their linux. Ubuntu is not doing anyone any favors with their "extended support programs".
Sigh ....
Davo