Misc enhancements

[Bubbler-4]

• Fix the tabbed pane to the bottom of the page
• Make it resizable
• Explainer (more like what HexagonyColorer does; draw paths over cells and extract the command sequence)
• Save explained code as image (SVG)
• Zoom in/out code area
• Custom codel size on PNG export
• Make test case separator more sane (do not require an empty line on its own for empty test case) ... or just make it more proper and use multiple input/output windows

[razetime]

would zoom in/out be done via scroll wheel?