Draft: GTK4_refactor

Starting the process of upgrading to use GTK4. This will be a large refactor with breaking changes across the whole application, so I am keeping the development isolated on this separate branch and will pull in changes made to master as needed.

I'm opening this draft merge request to provide both an easy way to see the changes made on this branch and master, and as a convenient place to allow discussions involving the refactor. Due to the scope of the changes needed, this will likely take a long time to complete.

At the time of the creation of this merge request, the cmake process has been updated to work with GTK4, but Remmina will still not compile.