Update the icon of the action opening the colormap dialog

[vallsv]

With the years of practice, i think the colormap dialog is mostly there to deal with the level of the image.

That is why i have changed this icon in Flint to help to deal with this specific use case. I still use the silx "style" icon at other places in Flint.

So i propose the use the icon i am actually using in Flint.

->

That's a small change which should help to use the 2 application at the same time.

BTW i have also reworked a bit the "style" icon in order to make it cleaner at low resolution. I also can update the one from silx the same way if you like.

Obviously if it's fine, i rework this icon with the orange instead of the green.