Ensure in both ConversationListView and GearyController that before
increasing the conversation window size, that the ConversationMonitor
actualy has more email to load. When an load has finished, check that
the list could acyually use some more, so if we get to the end of the
scrollbar, before that happens, more will still be pciked up.

This (hopefully) fixes the autoloading sometimes getting stuck and not
loading more.