Blender Git Loki

UI: Remove panel X axis closing code

Horizontal panel alignment hasn't been used for years, and we have no
plans to use it in the future. It adds a fair amount of complexity to
the panel code which makes adding features take longer.

This code removes the X closing flag, and all of the logic / variables
unused without it.

This commit includes a file subversion bump.

Differential Revision: https://developer.blender.org/D8601