Icons: solve disparity between ds4 and ds6 icon sets

[ianmcburnie]

There is now a huge disparity between ds4 and ds6 icon sets. In next major version let's think of ways to solve this.

Perhaps the easiest way is to just make the ds6 icons the default for ds4 too (let's face it, some of those old ds4 icons have not aged well at all), and move the old ds4 icons to a icon-legacy module for those teams that still need access to the old icons.

[ianmcburnie]

Done in 13.0.0