[FIX] #460 Read-only participants can edit using keyboard shortucts

Based on article:
http://ydtech.blogspot.in/2012/06/eclipse-editor-partially-editableread.html

Change-Id: I73a4f528bd7c28e6f446d25334b82b0e1b182a58
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1915
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>