Propagate Shift+PgUp/PgDown in alternate screen

I cannot express this any better than magiblot at the linked bug report:

    In normal conditions, Shift+PgUp/Shift+PgDn allow traversing the
    scrollback buffer. However, when a client application enables the
    alternate screen buffer, scrollback is not available. In this
    situation, Shift+PgUp/Shift+PgDn keystrokes could be propagated to
    the client instead of being discarded by Konsole.

BUG: 461150