Add basic side menu for layer selection

This _should_ be smart and not load expensive layers until they're rendered.

style.css

@@ -8,5 +8,24 @@ html, body {
   position: absolute;
   top: 0;
   bottom: 0;
-  width: 100%;
+  left: min(max(200px, 20%), 400px);
+  right: 0;
+}
+
+aside {
+  width: min(max(200px, 20%), 400px);
+  padding: 0 1em;
+}
+
+aside ul {
+  margin: 0;
+  padding: 0;
+}
+
+aside ul li {
+  list-style: none;
+}
+
+h1 {
+  text-align: center;
 }