[notifications applet] Fix two qml warnings about assigning [undefined]

Summary:
Fixes
[...]/NotificationDelegate.qml:51:26: Unable to assign [undefined] to int
by using the correct name of the enum value.
[...]/NotificationDelegate.qml:122:33: Unable to assign [undefined] to bool
by proper initialization of notification model data

Test Plan: No more warnings e.g. on notifications for new emails from akonadi.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11743