Introduce the refresh method

Introduce the refresh method

This adds tui_win_info::refresh and updates tui_source_window_base to
implement it as well.  This lets us simplify tui_refresh_all, removing
a check of the window type.

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

	* tui/tui-wingeneral.c (tui_win_info::refresh)
	(tui_source_window_base::refresh): New methods.
	(tui_refresh_all): Call the refresh method.
	* tui/tui-data.h (struct tui_win_info)
	(struct tui_source_window_base) <refresh>: New method.