qt: use rotating 3D cone as busy indicator

Raymarched union of 3D round box and cone SDFs.

Based on the work of Inigo Quilez, many thanks to him, as this would not be possible without his work.

I have learned a lot from his articles, I highly recommend reading them: https://iquilezles.org/articles.

I have uploaded the shader to ShaderToy (Qt stuff edited out): https://www.shadertoy.com/view/tXfXDN.

Since we do not use Qt Quick 3D, I guess this is the most ideal way to render animated 3D objects.

This was first suggested in this thread: !7079 (comment 482168). BusyIndicator is supposed to be very lightweight, and it is the case with at least the Qt Quick Basic style. However, it appears that KDE people used pie chart (among other things) in Breeze style up until very recently, which possibly caused the high CPU usage.

An alternative to this would be using BusyIndicator from the Basic style, and not respecting the default style (can be Breeze on KDE Plasma by default).

Request review @chub.