Convert appender and layout config to use Describable/Descriptor

[jvz]

Now that there are two different appenders (each with their own layout to configure), it would be great to change the configuration to use Describable/Descriptor classes for appenders and layouts. This will allow for each appender and layout type to have their own config.jelly views which will make more complex logging configurations much easier to display and configure in the UI.

As a bonus, this feature could provide either a generic UI that uses log4j plugin metadata to figure out which parameters are needed (rather hard), or at least provide for a good starting point for additional appender and layout types to be configured.