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/gdbengine.h

@@ -126,7 +126,7 @@ private: ////////// Gdb Process Management //////////
     AbstractGdbAdapter *createAdapter(const DebuggerStartParametersPtr &dp);
     void connectAdapter();
     bool startGdb(const QStringList &args = QStringList(), const QString &gdb = QString());
-    void startInferior();
+    void startInferiorPhase2();
 
     void handleInferiorShutdown(const GdbResponse &response);
     void handleGdbExit(const GdbResponse &response);
@@ -146,6 +146,8 @@ private slots:
     void handleAdapterStarted();
     void handleAdapterStartFailed(const QString &msg, const QString &settingsIdHint = QString());
 
+    void handleInferiorPrepared();
+
     void handleInferiorStartFailed(const QString &msg);
 
     void handleAdapterCrashed(const QString &msg);