Provide UI feedback when waiting for opened message to be retrieved

When user double-clicks a message, the Viewer will not appear until
the ItemFetchJob retrieves the message. This can however take some
time when the Session is blocked and during that time user does not
have any feedback on what is happening.

This patch makes use of the busy dialog provided by KMCommands, so
that users can at least see that something is going on.

REVIEW: 121091