Add new Javascript observer

Output render progress onto the rendered map using Javascript
A minimum period between updates can be specified

overviewer_core/data/web_assets/overviewer.css

@@ -86,7 +86,7 @@ body {
 }
 
 
-#link, #coordsDiv {
+#link, #coordsDiv, #progressDiv {
     background-color: #fff; /* fallback */
     background-color: rgba(255,255,255,0.55);
     border: 1px solid rgb(0, 0, 0);