applets/grouping: Clean up, fix warnings

List of commits:

applets/grouping: Remove anchors.fill to self

It was clearly a copy-pasta mistake when the code was being adapted and simplified from systray or desktop containment.

applets/grouping: Fix reading height content items

/usr/share/plasma/plasmoids/org.kde.plasma.private.grouping/contents/ui/main.qml:95:
TypeError: Cannot read property 'height' of undefined

applets/grouping: Reset anchors.fill before reparenting

If an item was parented and anchor.fill'ed, then it might complain in logs.

applets/grouping: Clean up QML/JS code and bump imports