Add basic support for video capture devices

Use the existing device discovery mechanisms to add support for video
capture devices. Mostly copy paste with adaptations.

Attempt to get a list of video capture devices from Solid, and
corroborate them with PS and Phonon.

Some changes for the platform plugin device access lists.

Extend the KCM module to display video capture devices. Currently
it doesn't suport testing them. Only tested by showing a video
device.

Add blank lines here and there.