API: add `GDBDebugger >> getParameter:` and `setParameter:to:`
...to get / set GDB internal parameters such as prompt. The only
complication here is that when a parameter is set by MI `-gdb-set`
command, the `=cmd-param-changed' notification is not sent. This
may or may not be a GDB bug.
To make this transparent to `libgdbs` clients, intercept all `-gdb-set`
commands and when sucessful, emit the event manually. This way, client
may rely on value change notification (`GDBCmdParamChangedEvent`) to
detect changes.