The placement of the start and end factor and mapping settings for
curves has been quite misleading for a long time. They were in the
"Bevel" subpanel, but they aren't related to bevel because they affect
curves with only extrusion and no bevel.
This commit moves these properties to their own subpanel, labeled
"Start & End Mapping".
Differential Revision: https://developer.blender.org/D8910
4e667ecef9