as it turns out, it is not possible to set pending breakpoints until
gdb has loaded as image. so add some hooks to enable adapters to trigger
the initial breakpoint syncing at the right time. do not add additional
states (say, InferiorPreparing), as it would just complicate things.

this includes:
- move the gdb ownership back to the engine (thus strip down the
  adaptors as far as possible)
- make gdb startup synchronous
- make adapter shutdown synchronous
- fix the state transitions relating to shutdown

rename GdbResultRecord into GdbResponse
merge cookie QVariant into GdbResponse
move debugMessage convienience function from individual adapters to base

remove shutdownInferior()
rename shutdownAdapter() into shutdown()
remove errorString()
rename isAdapter() into isTrkAdapter()

remove shutdownInferior(), rename shutdownAdapter() into shutdown()