fix engine control widget sensitivity
fix crash when changing the Audio System

When the i/o channel spinbox widgets are realized, they call
print_channel_count(). A "0" is formatted as "all available channels".
This causes the GTK::Entry inside the spinbox to emit a "changed"
signal which in turn triggers a parameter_changed(), [..], store_state(),
which calls into the spinbox again while it is being realized,
which makes gtkmm barf.

Start Ardour with JACK running (autoconnect to jack), Window ->
Audio/Midi settings -> initial no_control_notebook. Change
Audio System -> [control widgets are realized] -> glibmm std::exception