Backend optimistions

Learned a lot of new yesterday in a job interview. There are some new C++ standards that I wasn't aware of previously. The current backend code base of is still far from satisfactory and any improvement is welcomed. The first two changes should bring audio benefits too so it's a win/win.

This ticket is to do a review, clean up and address these issues.

Code Review

• Return Value Optimisations/Copy Elision. Was not aware about this before. The Qt API now makes a lot of sense now 🗡️
• Virtual destructors. When to use them, and when not to use them. I have been lazy on this and I'm sure something is wrong in the backend code

Research and potential code review

Unsure if these are needed. Do more study/research on these:

• propagate_const
• std::move