Disable observer and jsdebugger services when they're already part of Qt

As of Qt 4.8 the observer and jsdebugger services are part of Qt. This
means the following changes are necessary in Qt Creator:

* Do not link to the QmlJSDebugger library.
* Do not create JSDebuggerAgent and QDeclarativeViewObserver instances
  in the QmlApplicationViewer and use the new declarative_debug CONFIG
  option to enable those services in Qt. This is done automatically
  for debug builds.
* Point out the QML Debugging Library and the QML Observer are "Not
  needed" in the Qt options page.
* Change the label in qmake options from "Link QML debugging library" to
  "Enable QML debugging". It still remains as a way to enable this
  functionality in release builds or for QtQuick applications not based
  on Qt Creator's template.
* Rely on qmlviewer for debugging QML UI projects rather than the
  observer, which is no longer necessary.

Reviewed-by: Kai Koehne

src/plugins/qmlprojectmanager/qmlprojectrunconfiguration.cpp

@@ -134,6 +134,8 @@ QString QmlProjectRunConfiguration::observerPath() const
     if (!version) {
         return QString();
     } else {
+        if (!version->needsQmlDebuggingLibrary())
+            return version->qmlviewerCommand();
         return version->qmlObserverTool();
     }
 }