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/TaskPocketParameters.cpp

@@ -130,6 +130,7 @@ TaskPocketParameters::TaskPocketParameters(ViewProviderPocket *PocketView,QWidge
     updateUI(index);
 
     // if it is a newly created object use the last value of the history
+    // TODO: newObj doesn't supplied normally by any caller (2015-07-24, Fat-Zer)
     if(newObj){
         ui->lengthEdit->setToLastUsedValue();
         ui->lengthEdit->selectNumber();