Add Playground Equipment preset category (close #56)

Add `playground=playhouse` preset (which has >1000 uses, more than some existing `playground` presets)
Fix the filenames of playground presets to match their tags

data/presets/playground/playhouse.json

@@ -0,0 +1,17 @@
+{
+    "icon": "temaki-play_structure",
+    "geometry": [
+        "point",
+        "area"
+    ],
+    "tags": {
+        "playground": "playhouse"
+    },
+    "terms": [
+        "children's house",
+        "playground house",
+        "playhouse",
+        "Wendy house"
+    ],
+    "name": "Play House"
+}