Fix colors in TUI mode in MS-Windows build with ncurses

Fix colors in TUI mode in MS-Windows build with ncurses

The MS-Windows port of ncurses fails to switch to a color pair if
one or both of the colors are the implicit default colors.  This
change records the default colors when TUI is initialized, and
then specifies them explicitly when a color pair uses the default
colors.  This allows color styling in TUI mode on MS-Windows.

gdb/ChangeLog:
2019-03-14  Eli Zaretskii  <eliz@gnu.org>

	* tui/tui-io.c [__MINGW32__]: Include windows.h.  Declare
	ncurses_norm_attr.
	(tui_initialize_io) [__MINGW32__]: Record the default terminal
	colors in ncurses_norm_attr.
	(apply_ansi_escape) [__MINGW32__]: If a color in a color pair is
	"none", replace it with the default color recorded in
	ncurses_norm_attr.