QHeaderView - rename many spans classes and variables

QHeaderView - rename many spans classes and variables

In (SHA) b800d8b94a7861ecf8853621f6556fca186fb5b7 the span model
was replaced with a plain section model. The code however still has
variables and classes called someting with spans which would be
confusing for possible new readers of the code.

This patch cleans up most of it. It only renames classes,functions
and variables (and not any semantics or the public API).

Change-Id: I6ceb068c7317223f0d8e37f8032197f518d0174c
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>