Drop having a separate type for style options and make them all strings, which

allows formats to be expanded. Any styles without a '#{' are still validated
when they are set but any with a '#{' are not. Formats are not expanded
usefully in many cases yet, that will be changed later.

To make this work, a few other changes:

- set-option -a with a style option automatically appends a ",".

- OSC 10 and 11 don't set the window-style option anymore, instead the fg and
  bg are stored in the pane struct and act as the defaults that can be
  overridden by window-style.

- status-fg and -bg now override status-style instead of trying to keep them in
  sync.

window.c

@@ -872,6 +872,9 @@ window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 	wp->fd = -1;
 	wp->event = NULL;
 
+	wp->fg = 8;
+	wp->bg = 8;
+
 	TAILQ_INIT(&wp->modes);
 
 	wp->layout_cell = NULL;