Visualize hot (on panel) sensors that became stale

When a hwmon module was unloaded or crashed any sensors it had that were
also pinned to the panel (i.e. a 'hot sensor') retained the value
they had last, hiding the failure.

To fix that, this first clears all hot labels, replacing them with a
placeholder, before checking the new sensor data.  The placeholder
chosen is ⚠ (\u26a0 warning sign), after some experimentation with other
alternatives.

For consistency, the placeholder used on initialization is also
changed.

freon@UshakovVasilii_Github.yahoo.com/extension.js

@@ -133,7 +133,7 @@ var FreonMenuButton = new Lang.Class({
             this._menuLayout.add(i);
         }
         let l = new St.Label({
-            text: '\u2026', /* ... */
+            text: '\u26a0',  // ⚠, warning
             y_expand: true,
             y_align: Clutter.ActorAlign.CENTER});
         this._hotLabels[s] = l;
@@ -416,6 +416,9 @@ var FreonMenuButton = new Lang.Class({
 
             this._fixNames(sensors);
 
+            for (let k in this._hotLabels)
+                this._hotLabels[k].set_text('\u26a0');  // ⚠, warning
+
             for (let s of sensors)
                 if(s.type != 'separator') {
                     let l = this._hotLabels[s.key || s.label];