Add `Midi Clock Type: Clock only` to Midi Preferences

[stauntonjr]

MIDI beat clock defines the following real-time messages:

clock (decimal 248, hex 0xF8)
start (decimal 250, hex 0xFA)
continue (decimal 251, hex 0xFB)
stop (decimal 252, hex 0xFC)

For many applications, only clock is desired and start/stop/continue messages should be filtered out (e.g. when a sequencer should not be triggered by DAW transport). This is a basic control that should be supported by the DAW and not offloaded onto the end user as an added responsibility to employ third-party hardware or software to accomplish the task.