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/ViewProviderPart.cpp

@@ -126,7 +126,7 @@ bool ViewProviderPart::doubleClicked()
 QIcon ViewProviderPart::getIcon() const
 {
     // the original Part object for this ViewProviderPart
-    auto part = static_cast<App::Part*>(this->getObject());
+    auto part = this->getObject<App::Part>();
     // the normal case for Std_Part
     const char* pixmap = sPixmap;
     // if it's flagged as an Assembly in its Type, it gets another icon