Add highlight style, title style, fputs_highlighted. Improve 'show style'

Have 'show style' and its subcommands using a style to style its output.
This allows the GDB user or developer to use 'show style' to visually see
with one command how all the current styles look like.

Add 2 new styles highlight style, title style and fputs_highlighted function.

Highlight style is used by fputs_highlighted to highlight the parts of
its char *STR argument that match a HIGHLIGHT regexp.
This (and the title style) will be used in a following patch.