fix shutdown paths

this includes:
- move the gdb ownership back to the engine (thus strip down the
  adaptors as far as possible)
- make gdb startup synchronous
- make adapter shutdown synchronous
- fix the state transitions relating to shutdown

src/plugins/debugger/debuggermanager.h

@@ -285,7 +285,7 @@ private:
 
     void cleanupViews();
 
-    void setState(DebuggerState state);
+    void setState(DebuggerState state, bool forced = false);
 
     //
     // internal implementation