LV2/midnam tweaks - fix race conditions

LV2/midnam tweaks - fix race conditions

* Emit signal once midnam was actually updated
* only re-read midnam if was it changed. This allows idempotent calls to
  read_midnam() - from the same thread.

At session-load a synth-plugin may load a soundfont in the background
and emit midnam_update() after the synth was initialized but before the
GUI thread connects to the signal. By making the call idempotent the
GUI can call read_midnam() after connecting to the signal to catch up.