OTContract: rename class to Contract
OTContract: rename files to Contract.{h,c}pp
Merge pull request #409 from toxeus/renameOTContract

OTContract: rename to Contract
Remove "Focusing" from OTClient/OTServerConnection

The function SetFocusToServerAndNym() always has to be called first to
ProcessMessageOut() to work properly.

I wrapped both into one now. OTServerConnection() still set some unnecessary
state that will be removed in further simplifications.
Remove unused private fields