Fix UI plugin package hierarchy

* Change package from org.perfclipse.editors to
* org.perfclipse.ui.editors
Add createScenario() method to ScenarioManager

- also reexport perfcake dependency
Add dependencies for org.perfclipse.ui

- Dependencies added:
    1. Eclipse GEF (org.eclipse.gef)
    2. Eclipse XML editor (org.eclipse.wst.sse.core,
org.eclipse.wst.sse.xml.ui, org.eclipse.wst.xml.core)
Add GEF Controller backbone structure

- this structure is not complete and needs to extend
Add design editor using GEF