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/ViewProviderTextDocument.cpp

@@ -91,7 +91,7 @@ bool ViewProviderTextDocument::doubleClicked()
 
         getMainWindow()->addWindow(
             new TextDocumentEditorView {
-                static_cast<App::TextDocument*>(getObject()),
+                getObject<App::TextDocument>(),
                 editorWidget, getMainWindow()});
     }
     return true;