first commit of "archiving" functionality.

currently, it uses an additional "inbox" database field to decide whether a thread/conversation should be displayed in "inbox"-only view.

the user can toggle "inbox"-only viewing mode using the toggle button on the dropdown menu.

things to improve:
- in order to restart the ConversationListLoader with a new filter (inbox == 0 or inbox == 1), the Activity is restarted. this is not visually pleasing.
- right now there's no way to "unarchive" a conversation
- the variable storing the boolean "view all"/"view only inbox" is not in a good place. additionally, long-term it might be better to move away from the "Toggle inbox view" approach and towards a "Inbox"/"All mail" split (more similar to gmail).