You can start from the QuickSegment slicelet example and customize the GUI further (add/remove any widgets anywhere): https://www.slicer.org/wiki/Documentation/Nightly/Developers/Slicelets#Slicelet_examples
This is how Slicer is used as a viewer/editor plugin from other applications. See for example this topic. You may contact the author of that topic to share best practices with you.