Add constexpr to Vector classes

This turns code like QDoublVector2D(1,1) into a compile-time
expression when using a C++11 compiler.

Since these classes are private some member functions were converted
into inlines to make maximum use of constexpr.

Keyword inline was added where function declaration was not exhibiting
the keyword but the declaration was using it. Under certain circumstances
this can prevent potential export/import issues on some compilers.

Change-Id: I1b35b03942c5939fbafe2819fc875ef96eacaf02
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>