Replace tunnel_waterway combo with structure_water radio

(closes #4384)

- adds the `layer` field so it works like the structure field on highways
- also defaults the tunnel type to `culvert` for waterways
- also fixed so that `layer` gets removed if user changes types or clicks delete

data/presets/fields/structure_waterway.json

@@ -0,0 +1,13 @@
+{
+    "type": "structureRadio",
+    "keys": [
+        "tunnel"
+    ],
+    "label": "Structure",
+    "placeholder": "Unknown",
+    "strings": {
+        "options": {
+            "tunnel": "Tunnel"
+        }
+    }
+}

data/presets/fields/tunnel_waterway.json

@@ -1,5 +0,0 @@
-{
-    "key": "tunnel",
-    "type": "combo",
-    "label": "Tunnel"
-}