Remove dead code.
Store the loading state explicitly.

We need to be able to distinguish it from content in order to reset it.
Reset the loading state of nodes that moved.

If we have requested data of a cell before a move, the corresponding reply
possibly contains the outdated index, and we then never request the content
for the new location. This fixes cells stuck at "Loading...", especially
with objects that are quickly created and destroyed, such as a QML
ListView.
Don't accept responses if we didn't ask for them.

This happens if nodes have moved (due to added/removed rows), but still
had pending requests. Re-request them for the new location instead.