Allow customization of the description window

[sesquialtera87]

The actual AutoCompleteDescWindow does a good job, but it would be nice to have the possibility of change or tweak something of it (for example add new actions to the nav-bar or create a totally different dialog). Now it's not possible, because class AutoCompletePopupWindow creates its own private description window without any possibility of intervention.

I have created the abstract class AbstractDescriptionWindow that collects some common logic of the description dialog, so that, when needed, one can create a custom window and register a factory in the AutoCompletion object.

Also addToNavBar methods are added into AutoCompleteDescWindow to allow insertion of actions into the nav-bar