Change PropertiesPanel interface.

* Rename it to IPropertiesPanel in accordance with new naming standards.
 * Do not derive from Context. That is not needed.
 * Add an icon() method.
 * Fix all usages of this interface.

Reviewed-By: dt

src/plugins/projectexplorer/projectwindow.h

@@ -52,8 +52,8 @@ QT_END_NAMESPACE
 
 namespace ProjectExplorer {
 
+class IPropertiesPanel;
 class Project;
-class PropertiesPanel;
 class ProjectExplorerPlugin;
 class SessionManager;
 
@@ -195,7 +195,7 @@ private:
     QWidget *m_projectChooser;
     QLabel *m_noprojectLabel;
     PanelsWidget *m_panelsWidget;
-    QList<PropertiesPanel *> m_panels;
+    QList<IPropertiesPanel *> m_panels;
 };