Dismissable: Increase threshold to 60%

A number of users have complained that they currently land up deleting
note accidentally because of the swipe gesture.

lib/widgets/icon_dismissable.dart

@@ -37,5 +37,10 @@ class IconDismissable extends Dismissible {
               ),
             ),
           ),
+          dismissThresholds: {
+            DismissDirection.horizontal: 0.60,
+            DismissDirection.endToStart: 0.60,
+            DismissDirection.startToEnd: 0.60,
+          },
         );
 }