Implement a reliable port selection API (official pull request against winks branch)

The current RtMidi implementation has a race condition that may be very annoying in certain cirumstances.

Steps to reproduce the problem:

1. add several virtual and/or usb devices
2. Start a RtMidi application with interactive port selection
3. After the Application presented the port list to the user, remove the removable device in the list from the system (delete or unplug it).
4. Select the a device that is below the unplugged device in the list.
5. RtMidi will fail or open the wrong device.

The current patch set contains a suggestion for a reliable API.
The main problem that shall be addressed is that the current API does not store port descriptors together with the meta data (port names/port numbers). Thus, both can get out of sync.

The patches are based both on master and winks include:

• Winks patches as well as reverting them to avoid problems synchronizing with the master branch.
• some code reorganization (extract common API into a separate class)
• Add a port descriptor class (With a more flexible port naming interface).
• Add port descriptor API: getPortList, openPort(PortDescriptor &, const std::string&), getPortDescriptor()
• Add Example programs.
• Add an automatic test case for all backends that support virtual devices. This has been inspired by the automatic loopback test case of the node-midi project.

I came across the following questions:
• What's the purpose of class RtMidi? Is it used directly?
• RtMidiIn and RtMidiOut serve as API containers. Why do you use virtual inline functions, here? I'd suggest to remove the virtual classifier from the member functions and declare them only in RtMidi if they are used both by RtMidiIn and RtMidiOut. This would render CommonMidiApi useless and I would reintegrate it into MidiApi. Otherwise the inline functions should be marked extern to avoid linker errors.
• There is no function call ot set or get the current API object from RtMidi(In|Out). This and the fact that the implementation classes Midi(In|Out)(Alsa|Jack|WinMM|WinKS|Core|Dummy) are not documented separately suggest that these classes are private to the RtMidi library. Is that right? In this case I'd suggest to remove these classes from RtMidi.h. Otherwise I'd suggest to add two functions to set and get the API object that is used by the RtMidi(In|Out).
• Maybe… probably… I might add a container API that allows to collect several APIs into one common API.