1. do more to ensure that we do not call MidiSurface::begin_using_device()
multiple times without ::stop_using_device() in between. This reduces the risk
of duplicate signal handler connections being made (it might even eliminate it).
2. Notify all control surfaces when MIDI connectivity is established AND
disestablished. This gives them a chance to update their notion of their
current connection state. This can be important with JACK across zombification,
but also likely across backend stop&start.
These changes currntly only impact classes derived from MidiSurface but
something equivalent is required for all control surfaces
* reserve "probe" to actually probe for devices
* use separate probe for libusb and MIDI port devices
* use "available" to check if surface can be used
* allow both methods to be NULL
* remove unused ControlProtocolDescriptor* argument
Most surface just return `true` for available.
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
libardourcp and now libardour_midisurface are not control
surfaces, but helper libraries for those.
They need to be deployed to the library folder (shared between
ctrl surfaces) and not scanned as ctrl surfaces at runtime.
