Beast improvements and vflib compatibility module work

* Add CallQueue vflib compatibility class
* Use run instead of run_one
* Merge BindableServiceQueue into CallQueue
* Take BEAST_VARIADIC_MAX into account
* Fix license headers as suggested by Vinnie
* Remove obsolete comment
* Add ManualServiceQueue
* Add ManualServiceQueue to beast_vflib include
* Move static unit test variables of header only classes to module cpp
* Remove no longer used mutex member
* _VARIADIC_MAX maxes out at 10
* Correctly apply BEAST_VARIADIC_MAX
* Merge BindableServiceQueue into CallQueue
* New GuiServiceQueue and its JUCE dependency
* Fix leftover merge errors
* Fix CallQueue unit test
* Don't use bassert for better CI support

beast/threads/impl/ServiceQueue.cpp

@@ -179,6 +179,12 @@ void ServiceQueueBase::enqueue (Item* item)
         waiter->signal();
 }
 
+bool ServiceQueueBase::empty()
+{
+    SharedState::Access state (m_state);
+    return state->handlers.empty();
+}
+
 // A thread can only be blocked on one ServiceQueue so we store the pointer
 // to which ServiceQueue it is blocked on to determine if the thread belongs
 // to that queue.