Introduces 'non-exclusive' emulator-space keyboards.

i.e. sets of keys that don't amount to an entire keyboard in the modern sense. Experimentally used by the Master System for its reset key.

Machines/MSX/Keyboard.cpp

@@ -57,5 +57,5 @@ uint16_t MSX::KeyboardMapper::mapped_key_for_key(Inputs::Keyboard::Key key) {
 		default: break;
 	}
 #undef BIND
-	return KeyboardMachine::Machine::KeyNotMapped;
+	return KeyboardMachine::MappedMachine::KeyNotMapped;
 }