Rework app into a class

This simplifies usage, goes from

	app['Foo']
to
	app.Foo

Also makes static analyzers able to detect invalid attributes as the set
is now limited in the class definition.

As a bonus, setting of the default field values is now done in the
constructor, not separately and manually.

tests/metadata/org.adaway.pickle

@@ -2290,4 +2290,8 @@ NsS'Update Check Name'
 p483
 NsS'Vercode Operation'
 p484
-Ns.
+NsS'added'
+p485
+NsS'lastupdated'
+p486
+Ns.