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
Merge pull request #411 from OttoAllmendinger/pr-unfocus

Remove "Focusing" from OTClient/OTServerConnection
UserCmdGetBoxReceipt: remove Angel