Better emoji picker handling for reactions

While this is much better it still has two small issues (which are more noticeable when there are several messages with reactions):

• Moving the mouse over the messages feels laggy (as the emoji picker is created and then destroyed when the mouse passes over each message that has reactions)
• Moving the mouse over the messages temporary increases the memory consumption (although the memory is freed soon afterwards once all the emoji pickers are destroyed)

Maybe it would be better to create the emoji picker only when the button to show it is actually clicked? Of course showing the emoji picker could still feel laggy, but it should be less common than just moving the mouse over a message. To address that maybe the emoji picker could be preloaded if the message was hovered for more than two seconds or something like that.

Originally posted by @danxuliu in https://github.com/nextcloud/spreed/issues/7326#issuecomment-1122256628

Start from the message reaction UI that creates and destroys the emoji picker while hovering over messages. Reproduce the lag and temporary memory increase with several messages, then compare hover behavior with opening the picker via its button. Done means hovering no longer needlessly creates pickers and opening reactions remains usable.