AboutPage,AboutItem: Fix and rework avatar loading controller
AboutItem: Remove superfluous height bindings depending on visibility
FormLayout does not account for hidden items, so resetting height in such way is not needed.
AboutItem: Remove leftover/unused loop variable
Amends c4257f96
AboutPage,AboutItem: Fix and rework avatar loading controller
It was completely broken because of a race between Repeater::count and actual items being available as children to iterate over. Regardless that was an inside-out twisted data flow, and should be replaced by a simple top-level property with a straightforward JavaScript array aggregator and a little helper function to reuse it in person delegate.
A new exposed property provides developers with means to control the defaults.
Potential area for improvement: both old and new code only detects avatar URLs of authors, but it should probably also account for credits and translators?