- Added a FilterViewportButton class to the UIComponent file, which shows/hides
the FilterViewport whenever it's clicked
- Thus, the DataViewport can now fill up almost the entire window

Tabs now work reasonably well when right-justified.

Border is now a square (except for upper-right corner).

Changes to the DataThread, SourceNode, and UIComponent classes allow data
threads to disable callbacks if they are no longer receiving input. So
far this has only been tested with the Intan Board, but it works really well.
Unplugging the board in the middle of acquisition allows the thread to exit,
and callbacks to be disabled, without any seg faults.