Add 'True Origin' placement mode

[j9liu]

Closes #426.

This replicates the "True Origin" placement mode we have in Cesium for Unreal. This will be helpful for users with non-georeferenced tilesets.

I rearranged the CesiumGeoreference's Inspector UI so the order was more intuitive. Here's what it looks like on both modes:

Cartographic Origin	True Origin

[j9liu]

Some UI tweaks:

• Disable "Place Origin Here" for true origin. I think it's going to be more confusing if it's made available
• Made "Origin Mode" group

Result:

[kring]

Looks great, thanks @j9liu!