amend 7be3c88a, update rec-en color
ArdourButton: suppress visible response if no action is defined.

This fixes issues where the button is used as Label
(currently track-numbers and meterbridge track-name)
explicit Button inactive state.

Ideally large parts of ArdourButton would become ArdourLabel,
which in turn is used by ArdourButton
mark "Label Buttons" as inactive