Merge the unicode branch into trunk.

Merge the unicode branch into trunk.

- src/support/unicode.[Ch]: new files with functions for converting
  to and fro ucs4, ucs2 and utf8.
- src/support/docstring.h: specialization of basic_string that
  holds a uint32_t internally.
- Several functions changed to use char_type instead of char or unsigned char.
- Qt3 and Qt4 sends ucs2 on to core
- Gtk sends ucs4 on to core
- Read and write utf-8 .lyx files.
- font_metrics and painter updated to handle ucs4 chars as input.
- Quite a bit of ugly compability code, conversion string->docstring, etc.
- Have fun...

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14661 a592a061-630c-0410-9148-cb99ea01b6c8