Allow to control threading & polling manually

[jcelerier]

• ALSA raw observer: unnecessary, does not create an ALSA object
• ALSA raw input: https://github.com/jcelerier/libremidi/blob/master/examples/poll_share.cpp
• ALSA seq observer: https://github.com/jcelerier/libremidi/blob/73e6ea40de1a1ee35f16022e70fecfb45ae4061d/include/libremidi/backends/alsa_seq/helpers.hpp#L143
• ALSA seq input: https://github.com/jcelerier/libremidi/blob/master/examples/alsa_share.cpp
• JACK observer : https://github.com/jcelerier/libremidi/blob/master/examples/jack_share.cpp
• JACK shared handler: https://github.com/jcelerier/libremidi/blob/master/examples/jack_share.cpp
• WinMM observer: irrelevant? now we pass handles and as much info as we can
• CoreMIDI: https://github.com/jcelerier/libremidi/blob/master/examples/coremidi_share.cpp

[jcelerier]

done! :-)