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).

src/interactive.c

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #include "labwc.h"
+#include "view.h"
 
 static int
 max_move_scale(double pos_cursor, double pos_current,