Fix many many problems in xtext.c related to character width.

* Rework multiline logic to use a GSList, not an array
* Remove all multibyte booleans; everything is potentially multibyte
  - Except for gtk_xtext_get_word(), which needs special attention
* Enhance pango work for bold and italics (attr_list -> attr_lists[4])
* Enhance gtk_xtext_strip_color () to save bold, italics info
* Involve emphasis information in all pixel-width calculations
* Rework find_x () to account for emphasis info in widths

Closes #891
Fixes #872