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.

Fixes #289
5 jobs for wip/289-folder-not-fully-populated in 25 minutes and 2 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #263095
fedora

00:09:23

passed #263096
ubuntu

00:09:57

 
  Test
passed #263097
flatpak

00:13:43

 
  Deploy
passed #263098
review

00:01:20

manual #263099
allowed to fail manual
stop_review