gui: use fixed icon for togglebutton

lib/gui.js

@@ -60,15 +60,12 @@ function (chroma, Map, Sidebar, Tabs, Container, Meshstats, Linklist,
     contentDiv.appendChild(buttons)
 
     var buttonToggle = document.createElement("button")
-    buttonToggle.textContent = ""
+    buttonToggle.textContent = ""
     buttonToggle.onclick = function () {
-      if (content.constructor === Map) {
-        buttonToggle.textContent = ""
+      if (content.constructor === Map)
         router.view("g")
-      } else {
-        buttonToggle.textContent = ""
+      else
         router.view("m")
-      }
     }
 
     buttons.appendChild(buttonToggle)