Constants in MIKMIDISystemExclusiveCommand don't work nicely in Swift.

[armadsen]

The #defined constants in MIKMIDISystemExclusiveCommand are bridged into Swift as Int32s. However, the properties they're to be used for, manufacturerID and sysexChannel, are UInt32 and UInt8, respectively.

This means a simple assignment in Swift produces a compiler warning, and they have to be converted to the correct types before being used:

mmc.sysexChannel = UInt8(kMIKMIDISysexChannelDisregard)
mmc.manufacturerID = UInt32(kMIKMIDISysexRealtimeManufacturerID)

We should convert these to real constants instead of #defines so they have a non-implicit type.