Add a footer / statusbar

[seanh]

To show error messages to the user when something goes wrong (e.g. not file could not be decoded to unicode), status messages about tasks tv runs in the background (e.g. sync support), and general info (Terminal Velocity version, notes dir, number of notes).

The statusbar doesn't have to be visible all the time, it can appear for a few seconds when it gets a message then disappear again.

It might be nice to integrate it with Python's logging module i.e. the statusbar would be a log handler.