Blender Git Loki

UI: Use property split for custom property edit popover

The panel looks out of place with the rest of Blender's UI and the
text is cropped. With property split turned on and a few smaller
tweaks these issues are fixed.

| Before | After |
|{F8700181}|{F8700183}|

Differential Revision: https://developer.blender.org/D8322