i18n property files should not need to be converted to ascii

It is currently required for the "legacy" property files, (those that are stored in resources/info/magnolia/ui/[modulename] that they be in an escaped utf format, ascii that looks like this:
menu.stkDialogs = \u30c0\u30a4\u30a2\u30ed\u30b0\u5b9a\u7fa9
Whereas the "new" property files, (thosse stored in resources/mgnl-i18n) can be in UTF-8 their native character sets.

It causes problems that only some files must be in this escaped format and it is an unnecessary extra hassle to have to convert them. The i18n implementation should be changed to accept the standard unicode character sets for all translation property files.

Currently the files often have to be translated with a tool on the command line like so:
"native2ascii -encoding utf8 messages_ja_old.properties messages_ja.properties"

Note: assigning to Greg for his review of the improvement, and suggestions on implementation.