taskmanager: Port PlasmaExtras.ScrollArea to PC3.ScrollView in GroupDialog

1. Port PlasmaExtras.ScrollArea to PC3.ScrollView, and port Repeater to ListView so only visible tasks are loaded.
2. Rework activeTask mechanism to fit ListView, so the highlight can be restored after opening the same textual list (findActiveTaskIndex()).
3. Drop updateSize() function because it's more distinct to use size bindings, and the text width is updated by updateMaxTextWidth().
4. Drop attachModel() function and set model property directly in ListView.
5. Add isGroupDialog bool property to MouseHandler because ListView uses itemAt instead of childAt to get the item at the cursor position, so drag and drop still works.
6. Several hacks to prevent warning messages, including "Binding loop detected" and "trying to show an empty dialog".
7. Add iconMargin and labelMargin, so those properties can be shared between Task and GroupDialog.

BUG: 433907