Just write your code in a modular manner or use oop or whatever you young kids call it.

I usually write my software is in at least 2 seperate entites - backend for logic and functionality and then use GUI of choice for frontend. If you have to use a gui toolkit you just need to implement the "presentation" bit.

This idea of GTK=bad, QT = Good or QT=bad and GTK=good is a pointless argument.
Anyway GTK is still supported (as legacy) in maemo.

You have 2 options now and more rootfs bloat too.