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.

mode-tree.c

@@ -557,7 +557,7 @@ mode_tree_draw(struct mode_tree_data *mtd)
 
 	memcpy(&gc0, &grid_default_cell, sizeof gc0);
 	memcpy(&gc, &grid_default_cell, sizeof gc);
-	style_apply(&gc, oo, "mode-style");
+	style_apply(&gc, oo, "mode-style", NULL);
 
 	w = mtd->width;
 	h = mtd->height;