switchable between Tk (standard Tcl/Tk widget set) and 
  Ttk (Tile). Initial default widget set is Tk. Now, toplevel 
  widget classes are removed and defined as aliases. 
  For example, "TkButton" is an alias of the "Tk::Button" class. 
  Those aliases are replaced when switching default widget set. 
  "Tk.default_widget_set=" is the method for switching default 
  widget set. "Tk.default_widget_set = :Ttk" defines Ttk (Tile) 
  widget set as default. It means that "TkButton" denotes 
  "Tk::Tile::Button" class. And then, "TkButton.new" creates 
  a Tk::Tile::Button widget. Of course, you can back to use 
  standard Tk widgets as the default widget set by calling 
  "Tk.default_widget_set = :Tk", whenever you want. Based on 
  thie feature, you can use Ttk widget styling engine on your 
  old Ruby/Tk application without modifying its source, if you 
  don'tuse widget options unsupported on Ttk widgets (At first, 
  call "Tk.default_widget_set = :Ttk", and next load and run 
  your application). 
  This is one step for supporting Tcl/Tk8.5 features.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15618 b2dd03c8-39d4-4d8f-98ff-823fe69b080e