Test and use new conda gtk dependencies

Up until now we used conda gtk dependencies from third party channels e.g. `pkgw/label/superseded gtk3` and `pkgw-forge adwaita-icon-theme` which were only available for Linux and MacOS, since the official releases were outdated compared to these packages.

Now there are official releases for all dependencies in the main `conda-forge` channel which come with the added benefit of also being available for Windows besides Linux and MacOS. Test builds with these dependencies and update tests and documentation accordingly.