JBIDE-15317 - LiveReload script injection corrupted html encoding

Using explicitely a charset when reading local files.
Default charset used is UTF-8
Adding a combo in the server configuration editor to let the user choose her charset and override the default UTF-8 value
Fixed the "content-length" response header value
Also, the returned content is encoded with the charset provided in the request's
"accept" header. If no charset is given, the Charset.defaultcharset() one is used
but this preference is *not* stored.