Wicket migration code sprint

References

Getting started: https://dzone.com/refcardz/getting-started-apache-wicket

Migration references:

• Migrate from 1.4 to 1.5
• Migrate from 1.5 to 6.0
• Migrate from 6.0 to 7.0

Old branch migrating a 2013 GeoServer towards 6.0 and full diff against its baseline

Mass changes

• All missing generics
• ResourceReference -> PackageResourceReference
• ParamResourceModel -> StringResourceModel (the Wicket 7 class is fluent enough that we don't need ParamResourceModel anymore)
• PageParameters -> has been moved in a different pakage, the way to fill it is different

TODOs

• Check internationalization
• Check that WicketCallback changes did not break the callbacks (some are pretty important)
• Check that the PageParameters migration picked the right to<TargetType> vs toOptional<TargetType> form