Make source windows be self-updating

Make source windows be self-updating

This changes the TUI source window to register itself on the
source_styling_changed observable, and removes a bit of code from
tui-hooks.c.  This reduces the number of uses of the TUI_SRC_WIN
global.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-source.c (tui_source_window): New constructor.  Add
	observer.
	(~tui_source_window): New destructor.
	(tui_source_window::style_changed): New method.
	* tui/tui-hooks.c (tui_redisplay_source): Remove.
	(tui_attach_detach_observers): Update.
	* tui/tui-data.h (struct tui_source_window): Make constructor not
	inline.  Add destructor.
	(struct tui_source_window) <style_changed>: New method.
	<m_observable>: New member.