C2LC-681: Change the Scene Background dialog to list the backgrounds in 3 columns

Description

If we make the dialog wider, it will cover more of the scene, so you will be able to preview less of the backgrounds as you select them. We discussed with Sepideh and we will try 3 columns. It will cover some more of the backgrounds but makes the dialog easier to use.

Change the styling to make the columns consistent width and set the width so that longer names are wrapped over 2 lines.

How does 3 columns do at high levels of zoom?