Debugger: Debugger protocol maintenance

- Move DisplayFormat from watchhandler.h to debuggerprotocol.h
- Add/update a few comments about the use of the protocol enums
- Make decodeData take a DebuggerEncoding instead of an int

Change-Id: I50bed70a5da2e94da46e894bf9136bc14c9a1b23
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/gdb/gdbengine.cpp

@@ -3498,7 +3498,7 @@ void GdbEngine::handleThreadNames(const DebuggerResponse &response)
             ThreadData thread;
             thread.id = ThreadId(name["id"].toInt());
             thread.name = decodeData(name["value"].data(),
-                name["valueencoded"].toInt());
+                DebuggerEncoding(name["valueencoded"].toInt()));
             handler->updateThread(thread);
         }
         updateViews();