Test and use new conda gtk dependencies

[mpsonntag]

Up until now we used conda gtk dependencies from third party channels e.g. pkgw/label/superseded gtk3 and pkgw-forge adwaita-icon-theme which were only available for Linux and MacOS, since the official releases were outdated compared to these packages.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