Add sample to key= and all three key= targets

[alcomposer]

key= sets three opcodes to the same setting:

key=51

Is equivalent to writing:

pitch_keycenter=51
lokey=51
hikey=51

pitch_keycenter=sample is able to set the region to playback with the sample's own keycenter metadata. Thus it would make complete sense to be able to also write: key=sample

And also: lokey=sample hikey=sample

@paulfd mentioned the idea of key=sample: https://github.com/sfztools/sfizz/pull/362#issuecomment-673933031

These additional opcode's would work together with lock_ prefix suggestion: #13 and enable very clear logic, without adding complexity to SFZ format.