include: Break out view.h from labwc.h

IMHO it encourages better design (by making dependencies more obvious)
to have source file/header file pairs like view.c/view.h, rather than a
monolithic header like labwc.h with everything in it.

I don't think we need to break up all of labwc.h at once, but maybe we
can start pulling it apart bit by bit as it's convenient.

Also:

- Move "struct border" to ssd.h so that view.h can use it without pulling
  in all of labwc.h.
- Add a missing required #include within scaled_font_buffer.h (forward
  declaration of "struct font" is not enough).

include/ssd.h

@@ -55,6 +55,13 @@ struct wlr_box;
 struct wlr_scene_tree;
 struct scaled_font_buffer;
 
+struct border {
+	int top;
+	int right;
+	int bottom;
+	int left;
+};
+
 struct ssd_button {
 	struct view *view;
 	enum ssd_part_type type;