genPOI: support polylines and filled-in polygons

Add support for polyline POIs just like in the good ol'
Google Maps days. See #883.

The format after this commit is:
{ id, x, y, z, text, color, polyline/polygon: [{ x, y, z }, ...] }

Optional properties:
- common ones like "icon" and "hovertext"
- "strokeColor" (string),
- "fill" (boolean)
- "weight" (integer)

Docs not included.

Largely based on PR #1536 by @jsmienk.
Closes #1456, closes #1536, closes #1643.

overviewer_core/data/web_assets/regions.js

@@ -7,11 +7,10 @@ var world = "top";
 markersDB[groupName] = {
 	"raw": [
 		{
-		"fillColor": "#00FF00",
-		"fillOpacity": 0.2,
 		"strokeColor": "#FF0000",
-		"strokeOpacity": 1,
-		"polygon" : [
+		"strokeWeight": 2,
+		"fill": true,
+		"polyline" : [
 			{"x": 347, "y": 67, "z": 95},
 			{"x": 347, "y": 77, "z": 95},
 			{"x": 347, "y": 77, "z": 105},
@@ -19,7 +18,7 @@ markersDB[groupName] = {
 			{"x": 347, "y": 67, "z": 105}
 		]}
 	],
-	"name": "Regions",
+	"name": groupName,
 	"created": false
 }
 
@@ -31,4 +30,4 @@ markers[world].push(
       "displayName": displayName, 
       "checked": true
 	});
-*/
+*/