Blender Git Loki

Rename show_layout_ui > show_corner_split and remove from UI

This breaks API compatibility. However we are now grouping this setting
in the proper section (preferences.apps), so scripts had to update anyways.

So they may as well do it for the final name.

The reason to remove from the UI is that this is intended for app setup,
and as such it should not be exposed to final users until we have apps
better presented (for 3.1 hopefully).

Differential Revision: D12516