Fix ConversationMonitor sometimes not loading more from remote

If a FillWindowOperation didn't load a full amount of messages, it just assumed that there were no more to load. This is not true however when loading locally, the folder's vector isn't fully expanded, and it gets to the end of the vector.

This patch fixes the operation to also queue another fill if the monitor's folder message window is smaller than the folder's total message count.

See #289 (closed)