Although we are calling all of them light probes, there are a lot of
differences between them. This commit does the following:
* Prevent user from changing the probe type once added
* Unify "sphere" and "cube" probes into reflection cubemap
(as before you can switch between them from the probe UI)
To be done
==========
* Don't show add probe menus unless we are on Eevee
* Light probes panels should not be visible in Clay. Light probe objects
should not be visible in Clay viewport (nor on Cycles).
Notes
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* We need icons for the different light probes, and for lightprobes as a
whole (we are using RADIO for now).
