Refactor water-features

Dam, weir, lock:
* Add rendering for dam areas (supercedes and closes #227, based on code by
  @chrisfleming), dam and weir nodes, and lock_gate line
* Render weir in right layer (resolves #403)
* Change rendering of weir so it renders with small dashes
* Use marker instead of image for lock_gate node
* Add labels for dam nodes, dam areas, weir, and lock_gate
* Improve labels for dam lines and lock lines
* Remove labels for undocumented and less common waterway=lock

Groyne, breakwater:
* Render groyne/breakwater area the same color as groyne/breakwater way
  (resolves #893)
* Add label for groyne, breakwater, and pier (resolves #474)

Other:
* In general rewrite and clean up code

amenity-points.mss

@@ -18,6 +18,7 @@
 
 @standard-wrap-width: 30;
 
+/* Note that .points is also used in water-features.mss */
 .points {
   [feature = 'tourism_alpine_hut'][zoom >= 13] {
     point-file: url('symbols/alpinehut.p.16.png');
@@ -1484,18 +1485,6 @@
     }
   }
 
-  [feature = 'waterway_lock'][zoom >= 15] {
-    text-name: "[name]";
-    text-size: 9;
-    text-dy: 10;
-    text-fill: #0066ff;
-    text-face-name: @book-fonts;
-    text-halo-radius: 1;
-    text-halo-fill: rgba(255,255,255,0.6);
-    text-wrap-width: @standard-wrap-width;
-    text-placement: interior;
-  }
-
   [feature = 'leisure_marina'][zoom >= 15] {
     text-name: "[name]";
     text-size: 8;