Gui: Use getObject<T>() helpers in classes

This commit is generated using regex based find and replace:

```
s/[\w:]+_cast\s*<([^>]+)\*>\s*\(\s*getObject\(\s*\)\)/getObject<$1>/
s/[\w:]+_cast\s*<([^>]+)\*>\s*\(\s*([^)]*)\s*->\s*getObject\(\s*\)\)/$2->getObject<$1>()/
```

To regenerate if needed.

src/Gui/ViewProviderGroupExtension.cpp

@@ -98,7 +98,7 @@ bool ViewProviderGroupExtension::extensionCanDropObject(App::DocumentObject* obj
 
 void ViewProviderGroupExtension::extensionDropObject(App::DocumentObject* obj) {
 
-    auto grp = static_cast<App::DocumentObject*>(getExtendedViewProvider()->getObject());
+    auto grp = getExtendedViewProvider()->getObject();
     App::Document* doc = grp->getDocument();
 
     // build Python command for execution