Change interface of utf8scanner_t

The interface is changed from beginscan/endscan
into cleartoken/settokenstart/settokenend.

Also the type splittoken_t is renamed into splitstring_t
and the implementation is moved into directory "C-kern/api/string/".
Add prevlastcolumn to textpos_t

The name of some of the functions are also changed.
Move presentation/X11 into platform/X11