Allow creating scene objects without context

[falkoschindler]

There has been confusion about creating scene objects (#1203).

Normally we always need to enter the scene context like this

with ui.scene() as scene:
    scene.sphere()

or like this (after importing the class Sphere from nicegui.elements.scene_objects):

with ui.scene():
    Sphere()

But in case we already have a scene, it was unclear why we can't write

scene.sphere()

The reason is that scene.sphere is a static attribute. When sphere() is called, it looks into the slot stack to find its scene. But if we didn't enter a scene context, the sphere can't find it.

Of course, we could simply implement non-static factory methods that instantiate objects. But this would require lots of code duplication to mirror the class signatures and possibly docstrings.

This PR solves this problem by storing the current_scene when a scene is entered as well as when __getattribute__ is called for one of the static 3D object classes. Now the object initializer can simply use current_scene to get its own scene.

To summarize, these are the possible methods to place objects in a scene:

with ui.scene() as scene:
    scene.box()

with ui.scene():
    ui.scene.box()

with ui.scene():
    Box()

scene = ui.scene()
scene.box()

[rodja]

😎