Start using our new StackFrame and StackController classes as struts

Start using our new StackFrame and StackController classes as struts

* Source/DebuggerController.h: Add the stackController ivar
* Source/DebuggerController.m:
(init): Initialize the stackController
(dealloc): Release stackController
(stepIn:): Pop and push the stackController appropriately
(stepOut:): ditto
(stepOver:): ditto
* Source/GDBpConnection.h: -[stepIn] -[stepOut] and -[stepOver] now all return StackFrame objects
* Source/GDBpConnection.m:
(stepIn): Generate and return the frame
(stepOut): ditto
(stepOver): ditto
(createStackFrame): New private method
* Source/StackController.m+h:
(peek): New method
* Source/StackFrame.m+h:
(isShiftedFrame:): New method
(description): Implemented