Add an actionbar item for "add account" to the account list (if there's room)

This should be backported to 4.4
Ask SQLite for a simpler calculation when counting unread messages. The docs imply this is a decent perf win. I'm seeing at least a bit of one
Convert OpenMode from an Enum to static ints for perf improvement.
(Based on profiling of long folder list opens)

This should be backported to 4.4