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