Add keepBorder <theme> option and enable it by default

With the new keepBorder option enabled, the
ToggleDecorations action now has 3 states:

- the first time only disables the titlebar
- the second time disables the whole SSD
- the third time enables the whole SSD again

When the keepBorder action is disabled, the old 2-state
behavior is restored, e.g. the ToggleDecorations action
only toggles between on and off.

Fixes #813

include/ssd-internal.h

@@ -54,7 +54,8 @@ struct ssd {
 
 	/* The top of the view, containing buttons, title, .. */
 	struct {
-		/* struct wlr_scene_tree *tree;      unused for now */
+		int height;
+		struct wlr_scene_tree *tree;
 		struct ssd_sub_tree active;
 		struct ssd_sub_tree inactive;
 	} titlebar;