Background only: the UI hasn't been altered yet

To permit eventual retention of patching within cues (instead of using {n}th device, which may change between file loads)

Whilst it gets populated, and the UI reacts to user input, changes made are not saved.

Which emits whenever any input patch receives a new message. This allows
listeners to receive from all patched inputs easily, rather than having to
connect to them all individually. (And have to track when new patches are
added and handle when patches they're listening are removed.)

Sven if it doesn't match an available device

This is a bit of an odd solution, as whilst LiSP's configuration code detects
whether a config (sub)key has been added or had its associated value changed,
it doesn't pick up on key deletions.

Thus, we null the stored device name, and just treat any device patches with a
null'd name as if it doesn't exist.

Another odd quirk with LiSP's Signal class in that it doesn't like to be
connected to a slot belonging to a Qt5 widget, due to not being able to see
the slot's method signature. Hopefully the use of PyQt5's pyqtSignal class
instead doesn't cause any problems.

In the case that no patches are defined in `inputDevices` and `inputDevice`
doesn't exist in the config, don't create a default input patch.

Ditto for outputs.

1) So we don't throw an error about connecting to a non-existent port

whilst,

2) If it becomes available later, it should connect automagically (assuming rtmidi and ALSA backends)

Really need a wider Preferences UI!