Set initial metadata for ALSA MIDI raw and seq ports

[lucianoiam]

This patch extends #498 by setting initial metadata also for ALSA raw and seq MIDI ports.

@falkTX what do you think about renaming PortSetDeviceMetadata() to PortSetInitialMetadata() ? I think the latter better explains what that method does. There is another lowercase version jack_port_set_device_metadata() in alsa_midi_jackmp.cpp that should also be renamed, just in case.

[falkTX]

Renaming the function sounds good to me.
Note that you changed system:midi_capture_ name there to system_midi:capture_, which is not fine, please revert that section

[lucianoiam]

There was an existing naming inconsistency across the various MIDI drivers (some using system, some others system_midi), I cannot recall if I picked the convention from coremidi or winmme. If you think system:midi_* should be the standard I can also try to make all drivers follow that pattern. Or just revert this one.

[falkTX]

If there is no consistency somewhere, create a new ticket or PR for it, do not join it in this one.
the string before the : represents the client name, so it cannot just be changed at will

[lucianoiam]

Sure, sounds reasonable. Reverted change in last commit.

[lucianoiam]

Think I got it... alsarawmidi, coremidi and winmme are implemented as separate drivers, in contrast to raw and seq, hence the need for separate client names. I noticed that drivers are actually clients (is this correct?), for example JackAlsaDriver does this: alsa_rawmidi_new((jack_client_t*)this). There is no inconsistency at all. Sorry for the confusion.

[falkTX]

Thanks, merging now.