Refactor public interface to be less strict C and more python friendly

* Generally this means removing ref out parameters and instead returning
  values. In a couple of cases we will want to avoid copies in future
  either by returning const references (e.g. to the pipeline state which
  is immutable).
* At the same time, some pointless bool return values that were always
  true and didn't indicate errors have been removed. They can be added
  again if an error condition comes back.
* Some free functions still have out parameters as C linkage doesn't
  allow returning user types by value.
* The C# UI still invokes into C wrappers for all the C++ classes, which
  handle taking the return value and doing a copy into an out parameter
  still for compatibility.

qrenderdoc/Windows/Dialogs/LiveCapture.cpp

@@ -994,8 +994,7 @@ void LiveCapture::connectionThreadEntry()
       return;
     }
 
-    TargetControlMessage msg;
-    m_Connection->ReceiveMessage(&msg);
+    TargetControlMessage msg = m_Connection->ReceiveMessage();
 
     if(msg.Type == TargetControlMessageType::RegisterAPI)
     {