[ui] Improve 3D map scene config dialog

[nirvn]

Description

@wonder-sk , because my large screen couldn't even fit the 3d map scene config dialog anymore:

It's not ideal, but good enough to avoid documentation mess and backporting to 3.4. In 3.8, it might be worth re-thinking the dialog to use tabs, like labelling and mesh settings do. Have lights in its own tab, with a nice light icon, etc.

Checklist

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[saberraz]

Thanks @nirvn
You can mark this ticket as resolved in your commit:
https://issues.qgis.org/issues/20685

[wonder-sk]

Thanks for doing that! Agreed that later it should use tabs as the number of options is constantly increasing.

Also, one thing on my wish list is to be able to reuse Layer Styling dock for other purposes - such as 3D scene configuration or project properties

[DelazJ]

Thanks @nirvn
A help button leading to here or here would imho make the fix really complete (I checked but couldn't figure out where to add the changes so if anyone...)