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/net.osmand.plus.pickle

@@ -491,12 +491,16 @@ S'metadata/net.osmand.plus.xml'
 p178
 sS'Disabled'
 p179
-NsS'Update Check Name'
+NsS'added'
 p180
-NsS'Vercode Operation'
+NsS'lastupdated'
 p181
-NsS'Current Version'
+NsS'Update Check Name'
 p182
-S'1.9.5'
+NsS'Vercode Operation'
 p183
+NsS'Current Version'
+p184
+S'1.9.5'
+p185
 s.

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.

tests/metadata/org.smssecure.smssecure.pickle

@@ -772,4 +772,8 @@ NsS'Update Check Name'
 p227
 NsS'Vercode Operation'
 p228
-Ns.
+NsS'added'
+p229
+NsS'lastupdated'
+p230
+Ns.

tests/metadata/org.videolan.vlc.pickle

@@ -5610,14 +5610,18 @@ S'metadata/org.videolan.vlc.yaml'
 p1519
 sS'Disabled'
 p1520
-NsS'Update Check Name'
+NsS'added'
 p1521
-NsS'Vercode Operation'
+NsS'lastupdated'
 p1522
-S'%c + 5'
+NsS'Update Check Name'
 p1523
-sS'Current Version'
+NsS'Vercode Operation'
 p1524
-S'1.2.6'
+S'%c + 5'
 p1525
+sS'Current Version'
+p1526
+S'1.2.6'
+p1527
 s.