Change file validations in org.confetti.xml Wizards

Fix validaton and use decorators in ChooseFileWizardPage and
SaveFileWizardPage.
Rename Connections Preference page.
AddAssignmentDialog viewers look the same.
Main window shows active DataProvide type
Add getInformation() to DataProvider
DataProvider inherits from DataPersister

Separate read-only functionality from write functionalities in different
interfaces
Add org.confetti.fet.constraints plugin
Add ConstraintDialog
Change Import-Package to Require-Bundle