Fixed single page stepping in double-page mode for hotkeys.

This functionality has been broken for a while (since hotkey
modifiers have been fixed) and has now been introduced as new,
configurable hotkey.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@990 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Updated translation template.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@991 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Updated German translation.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@992 ef8490bc-f317-4725-81ed-e95a9aca1bcf