Debugger: Enabled state of "Threads:" label depends on thread combobox

...and making sure that items of a disabled combobox are visibly
disabled.

This prevents users from clicking on the Threads label before debugging
even started.

Change-Id: I59cc9b7e875fe8c126a9ee4221ff9d8322e4b1a8
Reviewed-by: hjk <hjk@theqtcompany.com>