add default mobwrite configuration
make the synchro server a common interface

the mobwrite bundle is a specific implementation of the synchronization
server, which may be replaced by another one (e.g. a Scala port of the
mobwrite protocol)
The interface is thus common to the entire \BlueLaTeX application and
has its own place in the common bundle.
hard-coded names are bad!

the bundle symbolic names is exposed by the `Bundle` class, thus it is
way better to get it this way instead of giving an hard-coded string