XMLEncoder should filter illegal XML characters

[dflorey]

JMTE may produce illegal XML output when using XMLEncoder.
XMLEncoder should use something like this to filter illegal Unicode characters (on top of current encoding rules):

int codePoint = text.codePointAt(i);
if (codePoint > 0xFFFF) {
    i++;
}
if ((codePoint == 0x9) || (codePoint == 0xA) || (codePoint == 0xD)
        || ((codePoint >= 0x20) && (codePoint <= 0xD7FF))
        || ((codePoint >= 0xE000) && (codePoint <= 0xFFFD))
        || ((codePoint >= 0x10000) && (codePoint <= 0x10FFFF))) {
    sb.appendCodePoint(codePoint);
}