Pharo9 utf8 encoding changes

[jbrichau]

Possibility to switch to Zinc UTF8 encoder/decoder in Pharo 9.

We keep the old TextConverter encoder/decoder as the default for now (and for those who need it in any case). The problem is that the old encoder (UTF8TextConverter) uses a stream on a String collection while ZnUtf8Encoder requires a ByteArray collection in the stream. Code changes are therefore often required. Seaside is not yet ready either but will switch at the some point.

Users can switch between the used encoders by using:
GRPlatform current setutf8CodectoDeprecatedTextConverter and GRPlatform current setutf8CodectoZinc