This will save us toLatin1() conversations in CppTools (which already
holds UTF-8 encoded QByteArrays) and thus loss of information (see
QTCREATORBUG-7356). It also gives us support for non-latin1 identifiers.

API-wise the following functions are added to Token. In follow-up
patches these will become handy in combination with QStrings.
    utf16chars() - aequivalent of bytes()
    utf16charsBegin() - aequivalent of bytesBegin()
    utf16charsEnd() - aequivalent of bytesEnd()

Next steps:
 * Adapt functions from TranslationUnit. They should work with utf16
   chars in order to calculate lines and columns correctly also for
   UTF-8 multi-byte code points.
 * Adapt the higher level clients:
    * Cpp{Tools,Editor} should expect UTF-8 encoded Literals.
    * Cpp{Tools,Editor}: When dealing with identifiers on the
      QString/QTextDocument layer, code points
      represendet by two QChars need to be respected, too.
 * Ensure Macro::offsets() and Document::MacroUse::{begin,end}() report
   offsets usable in CppEditor/CppTools.

Addresses QTCREATORBUG-7356.

Change-Id: I0791b5236be8215d24fb8e38a1f7cb0d279454c0
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>