Add basic implementation of MCStringGetCodepointAtIndex

Does not yet support UTF-16 with surrogate pairs!
Convert most users of MCFontMeasureText to use StringRefs

A number of CString users remain in block.cpp.
Add utility function MCU_r8tos

Required by the scrollbar drawing code.
Merge branch 'refactor-syntax_unicode_fontdraw' of https://github.com/runrevfraser/livecode into refactor-syntax_unicode_fontdraw
[[ UnicodifyGroup ]] Moved flag updating from MCGroup::load to MCGroup::save.