Fix issues with duration editor.

Add a DMDirc icon instead of java icon for editor window.
Stop doubling duration preferences.

Change-Id: Ide11244753c3fa86cb5914e1089929d385dc5545
Fixes-Issue: CLIENT-458
Fixes-Issue: CLIENT-459
Reviewed-on: http://gerrit.dmdirc.com/3207
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Build Manager