Implement StringCodec and others using CombinedChannelDuplexHandler

[RoganDawes]

It's a bit annoying to have to add both a StringEncoder and a StringDecoder node to the graph (on both the server and client pipelines!). These can be amalgamated using CombinedChannelDuplexHandler into a single node. Possibly the best way to deal with this would be to create a new tab in the palette for bidirectional codecs.

Start looking around here: https://github.com/sensepost/mallet/blob/master/src/main/java/com/sensepost/mallet/swing/GraphEditor.java#L216

This should probably apply to more than just the StringEncoder/StringDecoder, the idea applies to any "bidirectional" codec pairs.

[Tokarak]

I feel like taking this on.
Fixed link for reference:

mallet/src/main/java/com/sensepost/mallet/swing/GraphEditor.java

Line 216 in 31b3b6e

protocolPalette.addTemplate("StringDecoder",

edit:

mallet/src/main/java/com/sensepost/mallet/swing/GraphEditor.java

Line 196 in af477ea

protocolPalette.addTemplate("StringDecoder", IMAGE_ROUNDED, "rounded=1", 160, 120, createElement(xmlDocument,