Compress calls to `updateSize`

Summary:
Repeater+Flow has no reliable way to transactionize/batch insertion,
e.g. `Flow.positionCompleted` is useless in a function that can change
the Flow's size, and transition events like populated and similar
aren't available. Refactoring this to be a ListView isn't appealing
because it would mean the code loses the property of being reused for
both the bar and the popup, and therefore is well-tested already.

This brings down the number of `updateSize` calls when opening the
dialog for a group of five from five to one.

BUG:400364

Reviewers: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17006