Change tui_line_is_displayed to be a method

Change tui_line_is_displayed to be a method

This changes tui_line_is_displayed to be a method on
tui_source_window, now that it is obvious that it can only be called
for this type.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_line_is_displayed): Don't declare.
	* tui/tui-winsource.c (tui_line_is_displayed): Move to
	tui-source.c.
	* tui/tui-source.h (struct tui_source_window) <line_is_displayed>:
	Declare.
	* tui/tui-source.c (tui_source_window::line_is_displayed): New
	method.
	(tui_source_window::maybe_update): Update.