[CDF-431] Centralize MessageBundlesHelper calls: move class to cpf-core

	- added minor validation: check if a messages.properties was properly created/stored in cache dir; in the off-chance there isn't one, go ahead and create an empty one there
> "new byte[0] instead of new String().getBytes()" ? 

you are absolutely correct, will change that

> "doesn't this mean we're creating files for each dashboard regardless of whether they have translations" ? 

Yes, it does. 
This, however, has not changed, as is today's (read: pre-refactor) approach: any dashboard, when rendered, creates message files in their /tmp/.cache , regarding the ones placed at <plugin>/resources/lang ; see http://ibin.co/1jxBUNQ3lT6X or http://ibin.co/1jxJNR984Dcd ; those 2 sample dashboards do not have any translations ( Dashboards.i18nSupport calls );