Add empty!() for MIDITrack

[NeroBlackstone]

Is your feature request related to a problem? Please describe.
Sometimes we want to clear a track (remove all events in tracks), but currently no documentation of how this can be done.

Describe the solution you'd like
Implement empty!() for MIDITrack

like:

midi_file = load("sample.mid")
track1 = midi.tracks[1]
# clear all events in track 1
empty!(track1)

We can add empty!(t::MIDITrack)=empty!(t.events) in source code

Describe alternatives you've considered
Not implement this function, just add documentation about how to clear a track.

What do you think?

[Datseris]

Sure, you can add this overload