Icon design update to make visual shader toolbar more clear

[eobet]

• GridMinimap icon changed to remove the grid and instead add a minimap and make the map marker more clear (the motivation was that I constantly confused the mini map icon with the two grid setting icons in the visual shader editor).
• GridToggle & SnapGrid icons changed to replace the grid lines with dots, which closer reflects the default appearance of the visual shader editor and also makes the actual small corner symbols clearer (and reduces overall visual clutter/business which the lines caused).
• Both icons have had their color emphasis shifted to their corresponding small symbols and had the background elements toned down a bit more than previously using transparency.