Reorganize unicode string support in QString

Reorganize unicode string support in QString

Cleaned up preprocessor code to have a single definition for
QStaticStringData. A new qunicodechar typedef is introduced representing
a 2-byte integral type that can be used to represent a UTF-16 codepoint.

When QT_NO_UNICODE_LITERAL is not defined, QT_UNICODE_LITERAL converts a
US-ASCII string literal into a (native endian) UTF-16 string literal of
qunicodechar type.

Change-Id: I04822c4cdc0b240bc0fe113aba897348b7316932
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>