New feature style view

[jng]

The current feature style view suffers from the following issues:

• It is control-heavy. A lot of controls are created for each style rule within the style.
• We can't leverage GETLEGENDIMAGE-based previews without going into the actual style editor.

We can improve this part of the editor by having a grid-based view of the style rules, with lazy async background loading of GETLEGENDIMAGE-based style previews only for the styles currently visible on the grid.

This grid-based view should be much faster and responsive than the current apporach.