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