new load-more event

[fanckush]

Describe the feature request

currently tobottom emits when the list is reaches the end. This is useful for infinite lists but the user flow is broken because the user has to wait for the response before scrolling more.

It would be great of there was an event that would trigger when the list is almost at the end so that by the time the user reaches the end the new data was already fetched and the flow is continuous and smooth

[guerler]

A possible workaround is to use the range.start attribute on scroll and load additional data before the bottom is reached.