* 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.
