Patch adding a 'Paste Code' item to the 'Edit' menu, as well as hotkeys.

Using 'Paste Code' instead of the normal paste will remove prompts ('>>>
' or '... ') from the code, and also remove empty lines if pasting to a
shell window.

This allows easily copy/pasting code between shell and editor windows,
as well as easily pasting code from other applications (e.g. a web
browser) into IDLE.

This patch uses IDLE's PyParse.py for parsing code, and Tk's clipboard
API for interaction with the clipboard (it changes the clipboard's
contents, generates a normal paste event, and changes the contents back).

Test only on WinXP SP2 so far, needs to be tested on other platforms,
especially the clipboard interaction.