Support displaying Unicode text

Support displaying Unicode text

The current scheme, whereby we store the contents of our screen in an
array of char (two bytes in Java) with the upper byte used to encode
color and formatting information, is fundamentally incompatible with
Unicode, where the available encodings all either need more than one
byte per character or are variable-length encodings.

Instead, we use a new UnicodeTranscript class, which stores the
transcript row by row and includes facilities to handle combining
characters and East Asian wide characters.

We also need to make some minor changes to the code which displays text
on screen to support Unicode output.

Signed-off-by: Jack Palevich <jack.palevich@gmail.com>