[MlMt] Restrict Space Key scroll to only the current Mail

[Michael Nietzold]

Where:
- i have 3 Pane Layout
- i select an Message in the MessageList
- i see an eMail in Preview
- i hit many times the Space

what i see:
- if there are more than one message in MessageList then it shows the 
next message


what i expected:
- stay on the current message (and jump to the end)


my settings:
MailMate 1.12.4 (5594)

```
cat Custom.plist
{
         l = "loadImagesOnce:";
         "\UF72C" = "scrollPageUp:";
         "\UF72D" = "scrollPageDown:";
         " " = "scrollPageDown:";
}
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