[voicecall] Move provider plugins to a subdirectory for organisation
[voicecall] Remove AbstractVoiceCallManagerPlugin::pluginVersion().

This information was useless (never really incremented), and didn't serve much
real purpose (only used in a single debug message!).
Remove several copies of the license text.

The headers on each file are ample enough to give correct licensing.
Merge pull request #15 from rburchell/master

Small cleanups