change Call() and System() to discard the event

this also affects SendOSC() and SendDBUS().
it's easy enough to get the old behavior back by using +Call().
update docs of Process(), Call() and System()
improve documentation of mididings.util
document mididings.extra.gm
remove blank lines from Velocity() docs
improve Print() docs
move general description of splits

...from units to connections.
remove examples from "send_midi --help"
highlight current document in sidebar
add documentation of livedings and send_midi