SceneNodeData: allow access from wlr_surfaces

This replaces the old View.fromWlrSurface function and is more general.

This commit also moves the xdg activation request_activate listener to
Server as it has no reason to be in View.

river/Root.zig

@@ -195,7 +195,7 @@ pub fn at(self: Self, lx: f64, ly: f64) ?AtResult {
         break :blk null;
     };
 
-    if (SceneNodeData.get(node_at)) |scene_node_data| {
+    if (SceneNodeData.fromNode(node_at)) |scene_node_data| {
         return .{
             .surface = surface,
             .sx = sx,