Popup interface refactoring

[ACTCD]

This is an ongoing work and the last major refactoring before v5.0.

UPDATE: Since most tasks in 6.0 are merged into 5.0, the popup reconstruction will be carried out at the same time as the backend reconstruction. #805 (comment)

[ACTCD]

After the third refactoring of popup, it involved too many backend API changes, and that some "shim engineering" not only added complexity, also still suffered some limitations.

To save some unnecessary engineering and not release a only "surface" refreshed version, I finally decided to delay the UI refactoring and version 5.0.

I will first do heavy-duty backend reconstruction work to provide more appropriate APIs for the frontend and solve some existing complex issues.

These tasks were originally planned for milestone 6.0, but they will be merged into the 5.0 milestone with a delay of 5.0.

Sorry for the delay, but a more comprehensive update is worth a little longer, I will update further according to the progress.