Port away from ushort as char16_t and uint as char32_t

The use of ushort as the scalar type to mean QChar dates back to C++98 when char16_t and char32_t did not exist, yet. Now that they do, use of the C++11 character types yields self-explanatory APIs and less implementation work (casts).

Acceptance criteria:

• all Qt APIs that take ushort* or uint* to mean UTF-16 and UTF-32 strings are overloaded with char16_t* and char32_t* alternatives, respectively, and are deprecated according to project standards (some time in the future)
• all Qt code uses char16_t instead of ushort for UTF-16 character data and char32_t instead of uint for UTF-32 character data.