properties_view: Ignore result of sending SVG metadata across threads

parse_svg_file_future() is called in a cancellable async block. This means that after it's called it can be cancelled, which drops the receiver. When we fetch the metadata, we try sending the result to the dropped receiver.

This is expected behavior in this case - the receiver being gone at the time of sending the metadata is perfectly valid. However, previously we would cause the thread to panic. This didn't break anything but made for scary log message.

Since this is expected behavior we can use _ to tell the compiler that we don't care about the outcome of sending. This is valid in this instance because we will only get an error if the receiver is gone.