Use GTKFileChooserNative to improve flatpak sandboxing
The avatar selector uses a GTKFileChooser to allow the user to select a contact image. Using this API means that we need to open up the whole of the users home directory to a flatpak. If we switch to GtkFileChooserNative then we'll be able to significantly improve the sandboxing of the app without any degredation in user percieved functionality.