FCPPluginMessage: Rename member "parameters" to "params"

- Typical usage of the SimpleFieldSet in WOT FCP shows that people will
do a lot of "message.parameters.put(something)", so a shorter name is
better
- It is important to also rename the parameters of construct*() because
their JavaDoc says that the function parameters have the same names as
the class member variables and the reader should look up their meaning
there.