Improvements to `BackwardsPaginateUntilEvent` loading modal UI/UX

[kevinaboos]

Some miscellaneous issues needed to improve the loading modal added in #254.

• [x] ensure modal is only displaying as an overlay on a single RoomScreen widget instead of overlaid on the entire app window
  • Fixed by #258 (cc https://github.com/makepad/makepad/pull/598)
• [x] test behavior when closing a RoomScreen
  • ~~cannot test this until the modal is fixed to not occupy the entire screen ~~
  • Looks like the modal actually prevents the user from closing the RoomScreen, so this is not relevant.
• [x] Fix Makepad PortalList::smooth_scroll_to() to avoid drawing every item, and jump to 10 or so items beneath the target item to start, and then scroll upwards to the target item from there.
  • Fixed by #261