Add TUI border colors

Add TUI border colors

This adds the ability to change the color of the TUI borders, both
ordinary and active.  Unlike other styling options, this doesn't allow
setting the intensity, because that is already done by the TUI in a
different way.

gdb/ChangeLog
2019-12-01  Tom Tromey  <tom@tromey.com>

	* NEWS: Document new settings.
	* tui/tui-wingeneral.c (box_win): Apply appropriate border style.
	* tui/tui-win.c (_initialize_tui_win): Add border style
	observers.
	* tui/tui-io.h (tui_apply_style): Declare.
	* tui/tui-io.c (tui_apply_style): Rename from apply_style.  No
	longer static.
	(apply_ansi_escape, tui_set_reverse_mode): Update.
	* cli/cli-style.h (class cli_style_option) <add_setshow_commands>:
	Add "skip_intensity" parameter.
	<changed>: New member.
	<do_set_value>: Declare.
	(tui_border_style, tui_active_border_style): Declare.
	* cli/cli-style.c (tui_border_style, tui_active_border_style): New
	globals.
	(cli_style_option): Initialize "changed".
	(cli_style_option::do_set_value): New function.
	(cli_style_option::add_setshow_commands): Add "skip_intensity"
	parameter.  Update.
	(STYLE_ADD_SETSHOW_COMMANDS): Add "SKIP" parameter.
	(_initialize_cli_style): Update.  Create TUI border style
	commands.

gdb/doc/ChangeLog
2019-12-01  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (TUI Configuration): Mention TUI border styles.
	(Output Styling): Document new settings.

Change-Id: Id13e2af0af2a0bde61282752f2c379db3220c9fc