[wip] Add ToggleSnapToEdge

This allows to replace the usual maximize behavior that does not
respect gaps with SnapToEdge direction="center" which does respect
gaps. It can be bound to the SSD Maximize button and Title double
click handlers in rc.xml.

Missing:
- [ ] docs
- [ ] some enum for `<snapping topMaximize="yes|no">`
      like `yes|no|respect_gaps`

Ref: https://www.reddit.com/r/labwc/comments/1brvsj8/

src/view.c

@@ -1840,6 +1840,20 @@ view_edge_parse(const char *direction)
 	}
 }
 
+void
+view_toggle_snap_to_edge(struct view *view, enum view_edge edge)
+{
+	assert(view);
+
+	if (view->tiled != edge) {
+		view_snap_to_edge(view, edge, /*across_outputs*/ true,
+			/*store_natural_geometry*/ true);
+	} else {
+		view_restore_to(view, view->natural_geometry);
+		view_set_untiled(view);
+	}
+}
+
 void
 view_snap_to_edge(struct view *view, enum view_edge edge,
 			bool across_outputs, bool store_natural_geometry)