[GUI] notify the user on unavailable charset(s)

The user is now notified if a charset used in the project is not
available on his/her JVM. This should at least help unexperienced
developers who often forget to setup a proper character encoding (e.g
UTF-8) when working across different platforms.

Change-Id: I61f73bf58a055d977e7f2c86bf93e9c8b73cf981
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1397
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>