Make TUI react to "set style enabled"

Make TUI react to "set style enabled"

When the user toggles "set style enabled", the TUI should react by
redrawing the source window, if necessary.  This patch implements this
behavior.

No test because the TUI is generally not tested.

This version of the patch incorporates Pedro's patch to provide a
clean way to force the TUI to update the source window's contents.

2019-03-14  Pedro Alves  <palves@redhat.com>
	    Tom Tromey  <tromey@adacore.com>

	* tui/tui-winsource.h (tui_refill_source_window): Declare.
	* tui/tui-winsource.c (tui_refill_source_window): New function,
	from...
	(tui_horizontal_source_scroll): ... here.  Move some logic.
	* cli/cli-style.c (set_style_enabled): Notify new observable.
	* tui/tui-hooks.c (tui_redisplay_source): New function.
	(tui_attach_detach_observers): Attach or detach
	tui_redisplay_source.
	* observable.h (source_styling_changed): New observable.
	* observable.c: Define source_styling_changed observable.