Immediately blur input after typing valid character

This prevents the user from unknowingly having keyboard focus
on an input field after changing a key binding.

editor/js/Sidebar.Controls.js

@@ -36,6 +36,15 @@ Sidebar.Controls = function ( editor ) {
 
 			controlInput.dom.select();
 
+		} );
+		controlInput.dom.addEventListener( 'keyup', function () {
+
+			if(event.key.match(/^[A-Za-z0-9]+$/i)) {
+
+				controlInput.dom.blur();
+
+			}
+
 		} );
 
 		if( config.getKey( configName ) !== undefined ) {