QModelIndex: store quintptr instead of void*

QModelIndex: store quintptr instead of void*

Rationale:
1. Comparing pointers that don't point the same array is undefined behaviour, IIRC,
   and op== and op< did that.
2. The functions that cast to/from the storage type can't be constexpr. It makes
   more sense to have the quintptr functions be constexpr (they have a fighting
   chance to actually get passed something constant) than it is to have the void*
   functions constexpr. Thus, the storage type should be quintptr.

Also prepare op< to be constexpr-compatible.

Change-Id: I4b2d4a0ec8ca80d619d272bf07c57887cbd11c2f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>