ssd: Cosmetic cleanups

- Minimize includes in `ssd.h`
- Avoid repetitive `view->ssd.margin` pattern
- Use `struct ssd *` or `const struct ssd *` rather than `struct view *`
  where convenient

Part of the motivation is to make it easier to separate `struct ssd`
from `struct view` in a future commit.

src/workspaces.c

@@ -7,12 +7,12 @@
 #include <stdlib.h>
 #include <string.h>
 #include <strings.h>
+#include "buffer.h"
 #include "common/font.h"
 #include "common/graphic-helpers.h"
 #include "common/list.h"
 #include "common/mem.h"
 #include "labwc.h"
-#include "view.h"
 #include "workspaces.h"
 
 /* Internal helpers */