Fix modification date computation.

The modification date is currently updated whenever the Update() function is called, which is done for all tasks whenever a task is moved on the board. To avoid this:
 - Check for actual modifications, and update the modification date field only if changes are found.
 - Modifications to the position field are ignored unless the task was the one moved by the user.
 - For consistency, initialize modification date to creation date on task creation.
Highlight recently modified tasks.

Display a shadow around task when it was modified recently.
This is based on the date_modification field, and a configuration variable is used to specify the period during which tasks are considered 'recently modified'.