Consider using relm instead of gtk-rs

[jplatte]

It seems like relm could potentially make our code a whole bit nicer. Currently it seems to incomplete to seriously consider using it, but it might make sense in the future. This issue is meant to track relm's readiness for our use case.

List of issues I'd like to see resolved before considering a port:

• Tutorial
• First-class Glade support
• Menus (assuming this is referring to GtkMenu)
  Actually, we are using GtkPopoverMenu, which is a different thing.