Switch GUI toolkit
We should switch the GUI toolkit.
Currently we use avtk, which is a ntk based development of openav productions. ntk itself is a fork of fltk.
This brings some problems: No OpenAV productions project is under active development, especially not avtk. Same is true for ntk, fltk has some development but also a pretty small team. In the mid-term future there will be a switch from X11 to Wayland. FLTK already denied the feature request to support wayland, which means someday it wont work anymore for some users. All in all we have a complex dependency graph, which is maintained by too less developers. Avtk and ntk dont support any drag'n'drop, which makes #21 pretty hard to implement. To improve the maintainability of Loopp, there needs to be a seperation of the GUI from the application backend anyway, which requires some rework. Replacing the GUI in this process might reduce the workload in the long term.
So we should think about switching to another toolkit, in the best case a platform agnostic toolkit with drag'n'drop and good seperation of View and Controllers. It should support Wayland.
@rghvdberg proposed QT + QML, maybe there are some other possible solutions.