Introduce tui_win_info::clear_detail method

Introduce tui_win_info::clear_detail method

This introduces the clear_detail method and updates the various
subclasses of tui_win_info to implement it.  A subsequent patch will
remove tui_clear_win_detail, but that isn't done for now because at
this point it isn't readily obvious that the NULL check is not needed.

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

	* tui/tui-data.h (struct tui_win_info, struct tui_source_window)
	(struct tui_data_window, struct tui_cmd_window): Declare
	clear_detail method.
	* tui/tui-data.c (tui_source_window::clear_detail)
	(tui_cmd_window::clear_detail, tui_data_window::clear_detail): New
	methods.
	(tui_clear_win_detail): Simplify.