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/abstractgdbadapter.h

@@ -56,6 +56,7 @@ public:
 
     virtual void startAdapter() = 0;
     virtual void startInferior() = 0;
+    virtual void startInferiorPhase2();
     virtual void interruptInferior() = 0;
     virtual void shutdown();
     virtual const char *inferiorShutdownCommand() const;
@@ -80,6 +81,10 @@ signals:
     // Make sure to clean up everything before emitting this signal.
     void adapterCrashed(const QString &msg);
 
+    // This triggers the initial breakpoint synchronization and causes
+    // startInferiorPhase2() being called once done.
+    void inferiorPrepared();
+
     // The adapter is still running just fine, but it failed to acquire a debuggee.
     void inferiorStartFailed(const QString &msg);