Summary: These two should have been in D13821 but I missed them somehow.

Test Plan:
* Ran `qdbus org.kde.KWin /Effects loadedEffects`

* Ran the following Python script

```lang=python
import dbus

bus = dbus.SessionBus()

effects_object = bus.get_object('org.kde.KWin', '/Effects')
effects_iface = dbus.Interface(
    effects_object,
    dbus_interface='org.kde.kwin.Effects'
)

names = (
    'slide',
    'pizza',
    'cube'
)

supported_statuses = effects_iface.areEffectsSupported(names)

for name, supported in zip(names, supported_statuses):
    print("%s: %s" % (name, supported))
```

Got the following output:

```
slide: 1
pizza: 0
cube: 1
```

Reviewers: #kwin, mart

Reviewed By: #kwin, mart

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14002