Introduce initializer macros for QString- and QByteArrayData

Introduce initializer macros for QString- and QByteArrayData

This enables easier updating of those structs, by reducing the amount of
code that needs to be fixed. The common (and known) use cases are
covered by the two macros being introduced in each case.

Change-Id: I44981ca9b9b034f99238a11797b30bb85471cfb7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>