How to open a coredump?

Workarounds:
* Load Binary (or have it already loaded), then execute `core /path/to/core.file` in the GDB window
* start a new session with `/path/to/gdb --core=/path/to/core /path/to/binary`

Ideally there would be an option to do so directly in a session, and ideally gdbui would recognize that there is no process and disable the step into/out commands and friends with their shortcuts.

Start with the GDB window and session flow described in the issue, then trace how the existing `core /path/to/core.file` command and `--core` startup option are handled. Done means a session can open a coredump directly and disables step into, step out, and related shortcuts when no process is present.

Written by the indexing model from the issue text.