fix initial breakpoint setting

as it turns out, it is not possible to set pending breakpoints until
gdb has loaded as image. so add some hooks to enable adapters to trigger
the initial breakpoint syncing at the right time. do not add additional
states (say, InferiorPreparing), as it would just complicate things.

src/plugins/debugger/gdb/attachgdbadapter.cpp

@@ -82,6 +82,7 @@ void AttachGdbAdapter::handleAttach(const GdbResponse &response)
         QTC_ASSERT(state() == InferiorStopped, qDebug() << state());
         debugMessage(_("INFERIOR ATTACHED"));
         showStatusMessage(msgAttachedToStoppedInferior());
+        emit inferiorPrepared();
         m_engine->updateAll();
     } else {
         QString msg = __(response.data.findChild("msg").data());