Implement snes9x as emulator

[Makman2]

What needs to be done:

• Write an XML file to organize available emulators (location, invocation arguments etc).
• Modify snes9x
  • Use the Gtk.Plug mechanism to support UI parenting in the GameBox application.
  • Deactivate on-screen infos (like the one's that appear when loading a ROM or when saving a snapshot). Instead try to redirect these infos to GameBox.

Further steps: Use the modification as a submodule or supply a configuration script that writes the entry for this emulator inside the XML file, downloads, builds and configurates it (I'm in favor of the second version).

[Makman2]

For further steps: Maybe use an hybrid version of the two suggested approaches?