emacs: make show view a/A/x/X key bindings more consistent

emacs: make show view a/A/x/X key bindings more consistent

Modify the show view key bindings as follows to make them more
consistent:

'a' = Archive current message, then move to next message, or show next
thread from search if at the last message in thread.

'A' = Archive each message in thread, then show next thread from
search.

'x' = Archive current message, then move to next message, or exit back
to search results if at the last message in thread.

'X' = Archive each message in thread, then exit back to search
results.

The changes make the key bindings more consistent in two ways:
1) 'a'/'A' both advance to the next thread like 'a' used to.
2) 'x' operates on messages and 'X' on threads like 'a'/'A'.