Add a method for creating simple notifications

Summary:
Add a helper function to the daemon interface, which must be subclassed
by the implementations, which creates a simple notification.

For more complex needs, involving interacting with the notification, it
will still be nescessary to use KNotification directly, but this allows
for other future implementations to create a simple notification without
ifdef'ing the code.

Reviewers: kdeconnect, apol

Reviewed By: apol

Subscribers: kdeconnect

Differential Revision: https://phabricator.kde.org/D11104