Better UTF-8 support, including combined characters. Unicode data is now stored

as UTF-8 in a separate array, the code does a lookup into this every time it
gets to a UTF-8 cell. Zero width characters are just appended onto the UTF-8
data for the previous cell. This also means that almost no bytes extra are
wasted non-Unicode data (yay).

Still some oddities, such as copy mode skips over wide characters in a strange
way, and the code could do with some tidying.

TODO

@@ -69,11 +69,6 @@
   errors better
 - would be nice if tmux could be the shell (tmux attach, but hard link to tmux
   binary as "tmuxsh" or wrapper script?) -- problems with tty dev permissions
-- UTF-8 combining characters don't work. store in one cell - pointer to table
-  of chains. bit of a hack and would mean max of 65536 per screen. OR maybe
-  utf8 should work differently; could store as multiple cells, 1 of width >0
-  and n of width 0, then translate cursor indexes on-the-fly would need to
-  adjust all cursor movement and also handle different width lines properly.
 - support other mouse modes (highlight etc) and use it in copy mode 
 - set-remain-on-exit is a bit of a hack, some way to do it generically?
 - set-option should be set-session-option and should be overall global options
@@ -92,6 +87,7 @@
 - refer to windows by name etc (duplicates? fnmatch?)
 - a confirm-before command which asks "Are you sure? (y/n)" before executing
   command, eg bind-key "&" confirm-before "kill-window"
+- c&p is funny w/ UTF-8
 
 - document repeat behaviour and -r on bind-key
 - document status-keys