Better split view

[aleb2000]

This PR also updates some web-sys related dependencies to a newer minor version (simply because split-yew is already using the newer version).

• The split view is initially collapsed.
• The draggable gutter is always visible, so the user can always drag it wherever it wants anyway.
• On first run it will automatically resize to a 50/50 split.
• The split does not use a grid layout anymore but a flex one instead (Split.js does not support grid layouts, there is a split-grid version that does but my library currently does not support that). The grid layout wasn't doing much anyway.
• I did not add any buttons to collapse the split, but if you want it, that can be easily done by setting the "output_collapsed" state variable to true.

[ranile]

Thanks