Fix dbus export of property Device::supportedButtons

Summary:
When launching the touchpad kcm we see the following warning:
    Error on d-bus read of "supportedButtons"

And the Gui shows "Error while loading values. See logs for more
information. Please restart this configuration module".

By extending the unit test for reading back the properties through DBus
I noticed that it's not trivially possible to export QFlags to DBus. As
the QFlags can be implicitly casted to int, the property is changed to
an int value.

Test Plan: Test case adjusted, did not pass prior to change, passes now.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17472