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.
Updating the HTML docs with setIdleCommand().
Merge pull request #19 from remy-j-a-moueza/adding-idle-callback

Added support for an idle callback.