ISIS-771: comments to indicate why upgrade broken

... due to Wicket.Window.get() returning null in Wicket 6.12.0 and later.
ISIS-771: updated to Wicket 6.15.0

including Martin Grigorov's fix/workaround to handle change to Wicket behaviour as result of WICKET-5101.

In addition:
- fix to Isis' internals of reflectively invoking methods to ensure that any primitive parameters always get a non-null default value if not specified.
ISIS-773: IntelliJ-style regions

for example apps / archetypes.