Introduce make_visible method

Introduce make_visible method

This introduceds the make_visible to tui_win_info and overrides it in
subclasses as appropriate.  This allows the removal of the
tui_win_is_source_type, as it is no longer used.

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

	* tui/tui-wingeneral.c (tui_win_info::make_visible)
	(tui_source_window_base::make_visible): New methods.
	(make_all_visible): Make method call.
	* tui/tui-data.h (struct tui_win_info) <make_visible>: New method.
	(struct tui_source_window_base, struct tui_cmd_window): Override
	make_visible.
	(tui_win_is_source_type): Don't declare.
	* tui/tui-data.c (tui_win_is_source_type): Remove.