PartDesign/Gui: start to unify view providers code

- Moved all common code for part design features view providers to a base
   class.
 - Move shared code for Sketch based features to newly created
   ViewProviderSketchBased class
 - Add ViewProviderSketchBased to initialization
 - Make Pad and Pocket ViewProviders to use the shared code
 - Minor fixes to TaskFeatureParameters and some derived classes

src/Mod/PartDesign/Gui/AppPartDesignGui.cpp

@@ -37,6 +37,7 @@
 #include "Workbench.h"
 #include "ViewProviderPocket.h"
 #include "ViewProviderBody.h"
+#include "ViewProviderSketchBased.h"
 #include "ViewProviderPad.h"
 #include "ViewProviderChamfer.h"
 #include "ViewProviderFillet.h"
@@ -119,7 +120,8 @@ PyMODINIT_FUNC initPartDesignGui()
 
     PartDesignGui::Workbench                 ::init();
     PartDesignGui::ViewProvider              ::init();
-    PartDesignGui::ViewProviderBody      ::init();
+    PartDesignGui::ViewProviderBody          ::init();
+    PartDesignGui::ViewProviderSketchBased   ::init();
     PartDesignGui::ViewProviderPocket        ::init();
     PartDesignGui::ViewProviderPad           ::init();
     PartDesignGui::ViewProviderRevolution    ::init();