Initial work on EventFormatters.

These will be used to format events for displaying in a window.

Change-Id: I30381c2adc7bd77de486d6c0410a0a6395a7ea8d
Reviewed-on: http://gerrit.dmdirc.com/3835
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager