Queuing a watched channel's not-yet-downloaded videos previously meant
expanding the source and using its "Download N pending" button. Watched
source rows now carry a one-click "Download new" that loads the source's
items (even while collapsed) and queues everything not already
downloaded or in the queue, using the source's resolved kind/profile.
Extracts the shared, tested selectUndownloaded selector (also now backs
the expanded panel's pending list) and adds ensureSourceItems to load a
collapsed source's items without expanding it.
