Set limit on number of legend items

[nf-s]

Some layers show way too many legend items:

 {
      "type": "csv",
      "url": "test/localphotostories20092014.csv",
      "name": "ABC Photo Stories"
    },

Add a limit to number of items shown (and maybe a toggle collapse thing)

[KeyboardSounds]

Having too many legend items also makes things take ages to load, like this CSV file: https://drive.google.com/file/d/1Vi3RC940j9C0KpZBY-ubRd4yXcZ9Qrq5/view?usp=sharing

[nf-s]

This is fixed by #5426 and #5507