Remove struct tui_command_info

Remove struct tui_command_info

Like the previous patch, this removes tui_command_info in favor of
putting it elements directly into tui_cmd_window.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_source_disasm_command)
	(show_source_or_disasm_and_command): Update.
	* tui/tui-io.c (update_cmdwin_start_line)
	(tui_redisplay_readline): Update.
	* tui/tui-data.h (struct tui_command_info): Remove.
	(struct tui_win_info) <detail>: Remove command_info member.
	(struct tui_data_window) <start_line>: New member, from
	tui_command_info.
	(TUI_CMD_WIN): Add casts.