Linux, file format, and GUI improvements

Linux fixes - removed or wrapped most of the Windows-specific directory
encodings and made win32api (hopefully) optional

File format fixes - files are not processed as Numpy arrays anymore,
because Numpy doesn't like jagged/ragged arrays.  Instead, arrays are
transposed and padded with None before being passed to the plot command,
so "columns" can be accessed via a single index, regardless of whether
individual columns are the same length or not.  Demo data file added to
example directory.

GUI improvements - moved the "current directory" text box into the main
GUI window instead of only using it in file dialogs.  Made the default
program use the "toggled" window type, which allows the user to turn the
plot and/or spreadsheet off with a button press.  Added a demo program
showing how to use the Dialog functionality to create file dialogs.