Adds support for immutable and modifiable defaults system properties. (Deployment part pending).
Restores ConfigPropertyVetoException-s

This doesn't change anything semantically since the exceptions could
have already been thrown. They were just removed from the signatures in
an earlier commit and that might have been an omission during
refactoring.
Adds support for default configuration properties during name extraction.
Add support for a second defaults file.

The file is named jitsi-default-overrides.properties and it allows a
deployer (system administrator) to keep a separate set of locally
define properties, different from the ones the come with the client.

This commit also allows for zero length override property values.
Mionr: Use static method throgh class.
Merge branch 'master' of github.com:Metaswitch/libjitsi