Introduce ui.popup_edit

[evnchn]

Motivation

A popup edit is a long-request feature (#1125) but it was blocked by #2149

With #5612 this element can finally see the light

Implementation

• A very basic element
• Only handles show and hide, because all else is not relevant because we are not writing JavaScript

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Even tougher to pytest, I think. Let me try later.
• Documentation in website/documentation/content/popup_edit_documentation.py added in website/documentation/content/section_controls.py

[evnchn]

question 1 is it better to simply use https://nicegui.io/documentation/menu, since we don't care about the quasar JS-side modal values and stuff.

question 2 do we really don't care about JS-side modal values? It may be useful if we are putting this functionality in a table cell, for example.