QML UI: Use new JSON API for searching gpodder.net

This aligns with gPodder 4.x and also does things like sorting
results based on subscriber count, which makes more sense.
QML UI: Move filter to episode list header
QML UI: Also show spinner while episode list is loading

This gives a visual indication that even while the first few
items in the list are already visible, the list isn't fully
interactive (due to new items coming in regularly) yet.