Reimplement "jump to bottom" button using PortalList::is_at_end()

[jmbejar]

The "Jump to bottom" button visibility in the chat widget is currently controlled by having an extra empty element in the chat conversation list. When the associated PortalList renders this element it means we are "at the bottom of the list", so the button should be hidden.

In Makepad, a new PortalList::is_at_end() function was recently added. It theory, it should provide the bit of information we need here: if the list is scrolled at the end or not. The task is to replace our previous approach using this cleaner solution.

Note: We need first to bump to the latest Makepad version, which is an ongoing effort now. I will update this issue whenever is ready.

[kevinaboos]

apparently is_at_end() is not flawless yet, but it does appear to work better than prior attempts.

See our discussion here: project-robius/robrix#91 (comment)