title: Add a custom filter drop down for react-admin List view
body: <p>Question 1: I have a list view with a column for "Active":</p>
  
  <p><a href="https://i.stack.imgur.com/A7VVN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/A7VVN.png" alt="enter image description here"></a></p>
  
  <p>I want to allow the user to select Active/Inactive from a radio button group as a filter (or dropdown). I know react-admin provides filters for reference fields out of the box. What about a non-reference field, like "Active"? </p>
  
  <p>Question 2: Any way to add a clear button or "X" by the search field to quickly clear all filters? Currently, you have to select what's in Search and clear the text. Annoying having to use the mouse AND keyboard to clear filters. But, I'm guessing there's a way around it.</p>
  
  <p>Thanks.</p>