Add support for prepending elements
Add support for prepending elements

Although similar, prepending elements is not just a "mirrored" append;
besides caching the position and size of the added elements it is also
necessary to update the position of the rest of the elements, as it will
have have changed due to the prepended elements.

However, it is not necessary to show those elements to update their
position; it is assumed that the existing elements will not be modified,
so their position can be updated based on the height of the prepended
elements (which needs to account for a possible collapse of the margins
between the first existing element and the first prepended element).

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed