Block chooser search is not focusable when clicked on add button.

[iRohitSingh]

[JeffersonBledsoe]

Things I'd like to see from the solution for this:

• Focus is moved to within the block chooser after pressing the button. The search input at the top feels sensible (as was implemented by the original fix for this issue)
• Focus gets trapped within the block chooser until the chooser is closed
• There is an explicit button to close the block chooser that can be reached via keyboard (tab and screen reader)
• The button exposes an open/ closed state
• The picker can be closed via the escape key and focus returned to the button