rename ParameterChanged signal in Plugin to ParameterChangedExternally to reflect...

rename ParameterChanged signal in Plugin to ParameterChangedExternally to reflect its intent, and clean up the result.

The signal exists to notify listeners that something outside of the host's control (e.g. a plugin's own GUI for AU or VST)
has modified a plugin parameter. Previous code had strange feedback loops and ambiguous semantics.

Significant modification of LV2 GUI updating was required.

Still to be tested for feedback loop issues: AudioUnits