[Denemo-devel] Only show Prefs->Device Manager when JACK is active. Same for Internal Synth and its tab.

[Nils]

A few little gtk tasks that will improve the handling of Denemo by new users.

-Rename "Device Manager" to "Jack Device Manager" please
-Only show the Device Manager if "Jack" is chosen in "Misc"
-Only show the "Internal Synth" tab if Internal Synth is chosen in "Misc".

Its just a visibility thing but many people get confused by the fact that there 
is more than one place where you can choose audio-backends and drivers.

The advanced version would be to introduce a new column in the "Misc" tab and 
show the InternalSynth options or JackDeviceManager options there if its 
active. This gets rid of tabs and makes it clear which options belong to what 
Audio/Midi Setting.

Nils