Remove struct tui_point

Remove struct tui_point

struct tui_point does not help very much.  It is only used for
storage, and never passed between functions.  I think it makes the
code more verbose without any corresponding benefit, so this patch
removes it.

gdb/ChangeLog
2019-12-11  Tom Tromey  <tom@tromey.com>

	* tui/tui-wingeneral.c (tui_gen_win_info::make_window): Update.
	* tui/tui-win.c (tui_adjust_win_heights, tui_resize_all): Update.
	* tui/tui-layout.c (tui_gen_win_info::resize): Update.
	* tui/tui-data.h (struct tui_point): Remove.
	(struct tui_gen_win_info) <origin>: Remove.
	<x, y>: New fields.
	* tui/tui-command.c (tui_cmd_window::resize): Update.

Change-Id: I3f77920585b9ea9e2b4b189f3f3ae32d4da0c252