klipper: update model to support pinned items
Reason for the change
DeclarativeHistoryModel has two new properties that can be used to filter or prioritize pinned items.
- pinnedOnly: Whether to show pinned items only
- pinnedPrioritized: Whether to prioritize pinned items over other items except for the first item (current clipboard)
In the QML side an item can be pinned using model.pinned = true, or
unpinned using model.pinned = false.
This is the first step to add ability to pin items in Klipper.