FCPPluginClient: Fix documentation referencing a variable which has been
renamed
FCPPluginClient: Remove FIXME of splitting up send()

- The previous commits finished this :)
FCPPluginClient: Add missing <br> to JavaDoc
FCPPluginClient.send():

- Move local variable closer to where it is used
- Explain the assumptions that the message handler is not networked and
not a sendSynchronous() at the local variable which defines the message
handler
- Move the documentation which says that the message handler will be run
in a thread out of send() and to the backend function because it's not
visible in send() anymore that it is a thread - the function deals with
this now.