Thank you for the link.
As others have already pointed out, currently a lot of issues can be expected. And there is no easy fix to improve things.
In the last days I dug much in FCs Code, specially all the different uses of the file dialogs. Yes, it is complicated and every module does it a little bit different.
The big thing that bugs me with the gtk file dialog is, that it does not append the suffix to the file name, like the qt dialog does. And with the qt dialog it is annoying, that it does not reminds it's size if I have pulled it larger. I know that this are small issues, but from my point of view, software should be as handy as possible. Every little disturbance distracts me from the real work.
For now I'm happy, that I have managed to build my first little qt program, where I can play a little bit around with C++, qt and file dialogs. May be, some day I get to a good solution