element.delete() does not correctly delete an element

[falkoschindler]

Description

Noted by @dreusel on Discord and in #1500, the delete() method does not actually delete an element. It is merely used internally and can be overwritten to register some cleanup procedure, like e.g. here in ui.scene:
https://github.com/zauberzeug/nicegui/blob/main/nicegui/elements/scene.py#L187-L189

But instead of simply renaming it into _delete(), we should also think about a method for removing a single element, which could be delete(). In contrast to clear() (remove all children) and remove() (remove a single child) it could be called to remove the element itself.

So I propose the following:

• rename delete() to _on_delete()
• introduce a new delete() that removes a single element from the UI
• add (or extend) a section in the documentation explaining clear(), remove(), delete() and maybe _on_delete()

[falkoschindler]

Actually the title is incorrect: element.delete() does delete an element. But it is currently not the intended way, because it doesn't delete children and bindings.

[falkoschindler]

I started to work on the draft PR #1514.