Use Adwaita-like cursors

As this project moves more and more towards following the Adwaita design language, I think it's appropriate to talk about how the custom cursors relate to this. I think it would make sense from a consistency perspective to use black cursors with a white outline to match the Adwaita cursors, like this:

And for the select, resize and move cursors, just use the default (some styling would be needed for repeat/stretch). Of course, this would only be consistent on a GNOME system, but it would be impossible to make a set of cursors consistent with all platforms. So the choice is to either be consistent with one or none (or design separate sets for all platforms, but this is a lot of work). Because of the use of libadwaita, I think it's appropriate to choose consistency with Adwaita.

This design change would close #4447 (closed).