The Percolator class has been very hard to figure out, and has been a
source of confusion for users wanting to "hack" IDLE.

This patch makes Percolator a generally useful class which inherits from
Delegator. It also adds a new class, TkTextPercolator, which inherits
from Percolator and does all of the Text widget specific stuff.

The code has been refactored, cleaned up, and the in-code comments have
been improved. UndoDelegator.py, ColorDelegator.py and EditorWindow.py
have been updated as needed.

Also, the code using ColorDelegator in EditorWindow.py has been slightly
modified to work better with the Percolator.

I've done some testing (only on WinXP but nothing here is platform
specific) and this seems to work without a hitch.

This patch is required for the Squeezer and ShellLogger IDLE extensions
found on PyPI to be able to co-exist.