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/ViewProviderImagePlane.cpp

@@ -183,9 +183,7 @@ bool ViewProviderImagePlane::doubleClicked()
 
 void ViewProviderImagePlane::manipulateImage()
 {
-    auto dialog = new TaskImageDialog(
-        dynamic_cast<Image::ImagePlane*>(getObject())
-    );
+    auto dialog = new TaskImageDialog(getObject<Image::ImagePlane>());
 
     Gui::Control().showDialog(dialog);
 }