Detect if no audio Device is selected before trying to connect a server

[corrados]

See https://sourceforge.net/p/llcon/discussion/software/thread/ed7698b0cb/#13a9

It can happen that no Device is selected even though the dropdown is populated (e.g. if the audio interface you used in your previous Jamulus session is not curently connected).

If you then try to connect to a server, the main Jamulus window shows "TRYING TO CONNECT" message which is rather misleading.

The fix is of course to select input and output devices, but a more useful message would be helpful in this situation ("Please select audio devices for input and output before trying to connect").

[pgScorpio]

Another problem is that this also happens when there is a device selected but there are some undetected problems in it's ASIO setup !

In example when using FlexASIO, setting the number of input channels to 2 and your input device only provides 1 channel !

[corrados]

I have now implemented a check for correct audio device functionality. If the audio device does not work correctly, a warning message dialog is shown now:

[ann0see]

I think the message should start like this: "Please open Settings and check the[...]"
to clarify what to do for unexpected users.

[corrados]

Good idea. I just changed it:

[ann0see]

Great!