Updated the interpreter to escape certain special tcl characters.
Changed the addProtocolCommand method to be setProtocolCommand for consistency.
Added the ddoc file for others to generate the correct documentation.
Regenerated the documentation.
Updated the example.
Added support for an idle callback.

To make regular updates to the GUI while monitoring a subprocess or
retrieving data from another thread without freezing the entire application.

Note: For practical/implementation reason, the idle() methods have been added
to the UIElement class, but are not conceptualy bound to it.

Missing: HTML documentation upgrade. The gen-docs.sh script alone does not
generate documents with the proper styling.
Added support for an idle callback (2).

To make regular updates to the GUI while monitoring a subprocess without
freezing the entire application.

New version using Gary's feedbacks:
 - reverted the previous changes made to tkd/element/element.d,
 - renamed idle() to setIdleCallback(),
 - Remove the various overloads.