GtkFileDialog lacks ability to select folder or file

Builder requires a GtkFileChooser implementation which can handle both selecting a directory (when inode/directory filter is selected) or files when other file-types are selected for opening projects.

This is currently not supported in GtkFileDialog due to the async API being specific about one-or-the other.

I brought this up at the time of creation, but we didn't get any movement on the API churn.

This is blocking Builder from moving to GtkFileDialog for opening projects, which means we'll continue seeing "not nautilus" for opening files.