Introduce ui_file_style

Introduce ui_file_style

This introduces the new ui_file_style class and various helpers.  This
class represents a terminal style and provides methods for parsing and
emitting the corresponding ANSI terminal escape sequences.

gdb/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	* unittests/style-selftests.c: New file.
	* ui-style.c: New file.
	* ui-style.h: New file.
	* ui-file.h: Include ui-style.h.
	* Makefile.in (COMMON_SFILES): Add ui-style.c.
	(HFILES_NO_SRCDIR): Add ui-style.h.
	(SUBDIR_UNITTESTS_SRCS): Add style-selftests.c.