* ext/tcltklib/tcltklib.c: add methods to treat encoding mode.
* ext/tcltklib/MANUAL.eng: add description of TclTkLib#encoding,
  encoding_system, and so on.
* ext/tcltklib/MANUAL.euc: ditto.
* ext/tk/tkutil.c: fail to create a Tcl's list string from an
  array including multiple kind of encoded strings.
* ext/tk/lib/tk.rb: ditto.
* ext/tk/lib/multi-tk.rb: 2nd arg of _{to|from}UTF8 is omissible.
* ext/tk/lib/remote-tk.rb: ditto.
* ext/tk/lib/tk.rb: override TclTkLib#encoding and encoding= to
  use TkCore::INTERP.encoding and encoding=.
* ext/tk/lib/tk.rb: when "require 'tk'" and $KCODE=='NONE', check
  DEFAULT_TK_ENCODING to decide Ruby/Tk's system encoding mode.
* ext/tk/lib/tk/encodedstr.rb: check both of Tk.encoding and
  Tk.encoding_system. Tk.encoding has higher priority.
* ext/tk/lib/tk/optiondb.rb: ditto.
* ext/tk/lib/tk/spinbox.rb: ditto.
* ext/tk/lib/tk/validation.rb: ditto.
* ext/tk/lib/tk/namespace.rb: arguemnts for TclTkIp#_merge_tklist
  should be UTF-8 strings.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@8720 b2dd03c8-39d4-4d8f-98ff-823fe69b080e