api: Move haveGpgSupport() to GpgHelper
core: Recognize unknown protocol names better

Configuration files save protocol id as string version of PPID. This
makes sure the same strings can always be recognized as well.
api: Allow INetSocket usage without Buffer
qt4-gui: Use core to guess unloaded protocols
api: Add capability for conversation id usage
api: Get supported statuses from protocol plugins

Keep a mask of possible statuses for each protocol. Same handling as
for the capabilites mask.
qt4-gui: Hide unuseable global statuses