Implement incremental loading of messages

[octo]

The current approach places a user-configured "messages to load" cap on any given folder. While this works for a deliberate "what's new" use case, it is cumbersome for most other purposes. The reason is that a small limit is idea for performance, but not necessarily for usability.

At the user-level, several things are needed:

• Multiple message-count options
  • Max to load automatically (current value)
  • Max to retain once loaded
  • Max to load on trigger (i.e. a button to "load next XX")
  • Make theses account-level, consider folder-level overrides for IMAP
• Clickable MailboxScreen field for "Load next XX" messages
• Variation of that special field to handle mid-list gaps

At the protocol level, some improvements to range-based "get folder messages" code may be necessary, or may already be sufficient.

At the data model level, placeholder message nodes will be necessary in one of the two forms:

• Empty node for each potentially loadable message entry
  • Would be based on immediately available actions shown in the UI
  • New ones would be created after each selective load operation
  • More challenging to collapse at the UI level
  • May be easiest to support with the current data model, but potentially the least efficient
• Node signifying a range of possibly loadable message entries
  • Would correspond directly to available special UI fields
  • Would have a greater data model impact, especially for cache purposes
  • Would be more memory efficient

[octo]

Implemented