Blender Git Loki

Various improvements to Preferences buttons/layouts

* Header is now fully redundant. Added buttons to add studio lights under the Lights category.
* Removed redundant theme category dropdown
* Made the theme layout use layout flow, so it goes to single column when narrow, but multiple columns as you make it wider
* Made all the themes layouts consistent and all use property split fit with the rest of 2.8
* Fix UI Scale property so it doesn't flicker, by making it a number value rather than a slider - it's more correct this way anyway