Tidy up ActionEditorDialog and PropertyListCellRenderer.

Remove unused renderer, add super class, tidy javadoc.

Change-Id: Idd492b22d9ccf9b05e9386c4dcd60860b48e1a1b
Fixes-Issue: CLIENT-442
Reviewed-on: http://gerrit.dmdirc.com/3160
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>