QtCore: add constexpr to geometric classes

QtCore: add constexpr to geometric classes

This turns code like QPoint(12, 12) into a compile-time
expression, under a C++11 compiler, and allows to define
arrays of these types that end up in read-only memory,
just like PODs would.

Some constructors and QLine::pointAt() needed to be
adjusted to fit into the empty-body/only-return-expression
requirement for constexpr constructors/functions.

Change-Id: Id11ee2752c948930c3e40a91d1f6d7c97db7a373
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>