Add the `Edit` feature to the interactive stack.
When editing an object and replacing it with multiple objects, the
objects are inserted at the interactive stack location.
Add the `Info` button to show information about the stack level.
Separate the `Sort` feature into `MemSort` (sort by memory
representation) and `ValSort` (sort by value).
Add the `EchoNSp` to echo without spaces the way the HP48 does.
Fixes: #1031
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>
The tests for the interactive stack involved a change in the code to
show an object, in order to process test commands correctly and to not
overwrite the screen content with the stack.
Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>