[kcms/icons] Use OverlaySheet for icon size popup
Right now it's a custom pop-up which does basically the same thing as a Kirigami OverlaySheet, but it does not use that component, so as a result it is visually inconsistent.
This commit ports it to use Kirigami.OverlaySheet, making whatever minor layout changes to the UI are necessary to make it fit well inside the sheet.
Everything still works including focus and arrow key handling.
Here's how it looks now:
