Add prop for setting a fixed height for the dropdown

[davidsharp]

Using this we can restrict the height of the dropdown box, without setting a fixed height for component as a whole, meaning we can both render over components below, but also limit the ListView to be scrollable.

(Exists off the back of my previous PR #4)

[silesky]

Hey, would you be able to break this 'fixed height for the dropdown' feature into a separate PR (one that is distinct from the searchable ID feature?).

If not, you can close this PR and I can implement this feature by myself.