Don't call ::set_value() if the actual value has not changed.
e.g. MIDI-CC or integer controls. Moving the Bar-controller was able to
create events even though the actual value remained unchanged.
This check has to be done UI-side, since the underlying API is also used
for state-restore and automation. e.g.
"old value" (user-set) may be default "0", "new value" may also be "0" but
libardour still needs to send an event (a synth's internal state may not
default to "0")
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