#158 Add default right-button menu for entry and text widgets.

[Victor Wagner]

Most modern GUI system have a convention that
right-click of mouse on the widget brings up a menu
with context-related operations on this widget.

At least user expect Cut/Copy/Paste operations
availible on such menu of any editable widget.

Tk core doesn't provide such menus on entry and text
widgets.

Since incorporation of undo stack into core, undo and
redo operations should be added into this menu as well.

Attached file provides such bindings for Entry and Text
bindtags along with code which enables and disables
certain operation according to widget and system
clipboard state.

This file is a proper tcl package with namespace, but I
think that this feature really should go into Tk core.

[Victor Wagner]

Package to add Righ-click menus for entry and text