action: allow SnapToEdge to combine two cardinal directions

This patch adds `combine` argument to (Toggle)SnapToEdge actions.
This allows to snap a window to e.g. up-left by running two actions:
- `<action name="SnapToEdge" direction="left" combine="yes" />`
- `<action name="SnapToEdge" direction="up" combine="yes" />`

Then running `<action name="SnapToEdge" direction="down" combine="yes" />`
snaps it to left again. This behavior is almost the same as KWin, except
that snapping a up-right-tiled window to right doesn't move it to the
right-adjacent output, but makes it right-tiled first.

src/interactive.c

@@ -260,9 +260,8 @@ snap_to_edge(struct view *view)
 		view_maximize(view, VIEW_AXIS_BOTH,
 			/*store_natural_geometry*/ false);
 	} else {
-		view_snap_to_edge(view, edge,
-			/*across_outputs*/ false,
-			/*store_natural_geometry*/ false);
+		view_snap_to_edge(view, edge, /*across_outputs*/ false,
+			/*combine*/ false, /*store_natural_geometry*/ false);
 	}
 
 	return true;