BlockEditor: Preview column should be fixed width

[AnmAtAnm]

Currently, the blocklyPreviewContainer

adapts to the size of its children, the preview workspace, the block definition code preview, and the generator code preview. While the children are configured to scroll if their length is too wide, the lack of a size or limit to the blocklyPreviewContainer means the container grows before the scrollbar is revealed. Further, there is no trigger to resize the preview workspace, so the workspace can "grow" a right margin: