Set a default for offTheRecord in MorphWebContext (#376)

QtWebEngine 5.13 changed the default value of offTheRecord from false to
"is the given storage path empty?" This means that apps will dump all
cookies and site data on exit by default.

If someone is using the MorphWebContext as part of their app, they
expect the old behavior with offTheRecord = false. So we'll just set
that.

We have vendor-patched QtWebEngine to maintain the old behavior there.
I feel that this is still worth having to make future API breaks
explicit.

#347
qt/qtwebengine@6e28e76

src/Morph/Web/MorphWebContext.qml

@@ -28,6 +28,8 @@ WebEngineProfile {
     property alias incognito: oxideContext.offTheRecord
     readonly property string defaultUserAgent: __ua.defaultUA
 
+    offTheRecord: false
+
     dataPath: dataLocation
 
     cachePath: cacheLocation