Improve MultiSelect component

[robines]

Changes:

• Tweaked styling
• Replace +/- with more understandable "Select all" and "Unselect all" buttons
  • Instead of hiding these buttons when one side is empty, they are instead just disabled, which is a more UX friendly approach
• Set fixed height, which means no more layout shifts when selecting items.
• Replace individual filtering with a single filter input, which filters both available and selected lists.
• The "Select all" and "Unselect all" buttons now only move visible items
  • It's now trivial to select/unselect all items matching the filter, since you no longer have to individually click each item

[Snorre98]