Define exportable properties with 'var'

This is to fix this new gjs warning:

```
Some code accessed the property 'PanelWidget' on the module
'panelWidget'. That property was defined with 'let' or 'const' inside
the module. This was previously supported, but is not correct according
to the ES6 standard. Any symbols to be exported from a module must be
defined with 'var'. The property access will work as previously for the
time being, but please fix your code anyway.
```

Ref.: #198 #205

extension/widgets/categoryTotalsWidget.js

@@ -33,7 +33,7 @@ const Stuff = Me.imports.stuff;
 /**
  * Custom Label widget that displays category totals.
  */
-const CategoryTotalsWidget = new Lang.Class({
+var CategoryTotalsWidget = new Lang.Class({
     Name: 'CategoryTotals',
     Extends: St.Label,
 

extension/widgets/factsBox.js

@@ -40,7 +40,7 @@ const TodaysFactsWidget = Me.imports.widgets.todaysFactsWidget.TodaysFactsWidget
  * well as todays facts.
  * @class
  */
-const FactsBox = new Lang.Class({
+var FactsBox = new Lang.Class({
     Name: 'FactsBox',
     Extends: PopupMenu.PopupBaseMenuItem,
     _init: function(controller, panelWidget) {

extension/widgets/ongoingFactEntry.js

@@ -36,7 +36,7 @@ const Me = imports.misc.extensionUtils.getCurrentExtension();
  *
  *
  */
-const OngoingFactEntry = new Lang.Class({
+var OngoingFactEntry = new Lang.Class({
     Name: 'OngoingFactEntry',
     Extends: St.Entry,
 

extension/widgets/panelWidget.js

@@ -52,7 +52,7 @@ const Stuff = Me.imports.stuff;
  *
  * @class
  */
-const PanelWidget = new Lang.Class({
+var PanelWidget = new Lang.Class({
     Name: 'PanelWidget',
     Extends: PanelMenu.Button,
 

extension/widgets/todaysFactsWidget.js

@@ -33,7 +33,7 @@ const Stuff = Me.imports.stuff;
 /**
  * A widget that lists all facts for *today*.
  */
-const TodaysFactsWidget = new Lang.Class({
+var TodaysFactsWidget = new Lang.Class({
     Name: 'TodaysFactsWidget',
     Extends: St.ScrollView,