Scroll messages horizontally if necessary.

(needed for wide preformatted text blocks)
Enable workers for the Ace editor.
Added a background image for the wavy red underline for errors.
Highlight last executed/explained text, add line marker for error, and underline word at error position.
Updated message category icons for the new version of semantic-ui.
Fixed icon display in the Messages tab.
Collapse multi-line strings in banner messages using an accordion.