centralize gdb command result class checking

each command can have only one of two legitimate responses: "error" or -
depending on the command, and thus declared via a flag - "done" or
"running".
this is way nicer than sprinkling the code with else-ifs (where elses
are sufficient) and asserts all over the place - and silently failing in
release builds.

src/plugins/debugger/gdb/gdbmi.cpp

@@ -362,7 +362,7 @@ GdbMi GdbMi::findChild(const char *name) const
 //
 //////////////////////////////////////////////////////////////////////////////////
 
-QByteArray stringFromResultClass(GdbResultClass resultClass)
+QByteArray GdbResponse::stringFromResultClass(GdbResultClass resultClass)
 {
     switch (resultClass) {
         case GdbResultDone: return "done";