* ext/tk/lib/tk.rb, ext/tk/lib/tk/text.rb,
ext/tk/lib/tkextlib/iwidgets/scrolledtext.rb: remove adhoc check
of Ruby's features (use existence of some classes instead of
comparing with RUBY_VERSION)
* ext/tk/lib/tk/root.rb, ext/tk/lib/tk/autoload.rb: make TkRoot
(Tk::Root) unswitchable
* ext/tk/lib/multi-tk.rb: partial bug fix (still not work!!)
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15634 b2dd03c8-39d4-4d8f-98ff-823fe69b080e