Move make_visible method to tui_gen_win_info

Move make_visible method to tui_gen_win_info

This moves the make_visible method from tui_win_info to its base
class, tui_gen_win_info.  This allows the removal of another window
type check.

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

	* tui/tui-wingeneral.c (tui_gen_win_info::make_visible): Rename
	from make_visible.
	(tui_make_visible, tui_make_invisible): Rewrite.
	(tui_win_info::make_visible): Remove.
	(tui_source_window_base::make_visible): Update.
	* tui/tui-data.h (struct tui_gen_win_info) <make_visible>: New
	method.  Moved from...
	(struct tui_win_info) <make_visible>: ...here.