C++: Add utf16 indices to Macro and Document::MacroUse

In most cases we need to work with the utf16 indices. Only in
cppfindreferences the byte interface is still needed since there we read
in files and work on a QByteArray to save memory.

Change-Id: I6ef6a93fc1875a8c9a305c075d51a9ca034c41bb
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>