Expand material and gesture links (cfug#539)

* Expand material and gesture links

Fixes flutter/flutter#8369

* update API

* elevation now a double (cfug#538) (cfug#540)

gestures.md

@@ -87,12 +87,17 @@ lifecycle of the gesture (e.g., drag start, drag update, and drag end):
 To listen to gestures from the widgets layer, use a
 [`GestureDetector`](https://docs.flutter.io/flutter/widgets/GestureDetector-class.html).
 
-If you're using Material widgets, instead of using a gesture detector
-directly, consider using an
-[`InkWell`](https://docs.flutter.io/flutter/material/InkWell-class.html)
-to listen to taps, and a
+If you're using Material widgets, many of those widgets already respond
+to taps or gestures.
+For example,
+[IconButton](https://docs.flutter.io/flutter/material/IconButton-class.html) and
+[FlatButton](https://docs.flutter.io/flutter/material/FlatButton-class.html)
+respond to presses (taps), and
 [`ListView`](https://docs.flutter.io/flutter/widgets/ListView-class.html)
-to listen to swipes to trigger scrolling.
+responded to swipes to trigger scrolling.
+If you aren't using those widgets, but you want the "ink splash" effect on a tap,
+you can use
+[`InkWell`](https://docs.flutter.io/flutter/material/InkWell-class.html).
 
 ## Gesture disambiguation