Improve how the client triggers conversation auto-loading
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.
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