Simplify how we build record filenames

Refactor code dealing with save directory and filenames to be simpler.

Signed-off-by: Meg Ford <meg387@gmail.com>
Cleanup and fix main window UI packing

Remove some unneeded grids and wigets, and set the correct expand flags
so that the view resizes with the window.
Also, remove some hardcoded style, since gnome-themes-standard has a
stock style class to get the same visual effect.

Signed-off-by: Meg Ford <meg387@gmail.com>