[JBIDE-13182] replaced Base64Coder by JDK means to (de)code base64

Using DataTypeConverter to (de)code base64 instead of the copied Base64Coder class
we currently use. This copied class is considered as fork in Fedora and thus
prevents them from packaging it
(see https://bugzilla.redhat.com/show_bug.cgi?id=902017#c2).