Allow scrolling through the list of users in a room when selecting a user to mention

[ZhangHanDong]

once makepad supports setting max heights and percentage heights, we can adjust the size of the user list view to have a max height, and also make it a scrollable

[aaravlu]

I would love to solve this issue in makepad side.

[ZhangHanDong]

This makepad PR needs to be reviewed. #744

[kevinaboos]

This should now be possible by means of using the min/max values for Size::Fit and Size::Fill.