Change OpenTM2 config prefix to mandatory

Make the openTM2 tmprefix config (runtimeOptions.LanguageResources.opentm2.tmprefix) mandatory now. Currently it defaults to empty string. A alter script should change all values to "default" where the value is empty, then the config.default value should also changed to "default".

This does not affect already imported TMs, there the prefix must remain empty, otherwise the TMs in OpenTM2 could not be found anymore. Since the whole filename is saved in the LanguageResource this is although no problem.