I continued with implementation of getting the preview of entities while they get painted. If inspecting softwares like LibreCAD, the preview is visible to the user while he/she moves the mouse. What I have yet implemented, yet for line and circle, is that preview is available on mouse press and mouse move events together.
Another thing, we find that the algorithm that implements ellipse to be incorrect. Will be working on it today!
Considering the fact that Qt provides various entities as predefined objects in it, comparing it to LCv3, I doubt if we will be needing something like storage manager. This is to be scrutinized more.