todo_widget: Use colon `:` as task and description display separator.

Uptil now, a dash `-` was used to separate the task from its (optional)
description. But a colon `:` was used in the `/todo` command syntax to
indicate where the task ends and it's description starts.

To make the separator symbol consistent for both the syntax and widget,
`:` is used for both. It was chosen over `-` as `:` is encountered less
often in normal language, and so accidental usage of it when using the
`/todo` command syntax is less likely. It is also is impossible to
confuse with the bullet list syntax which is allowed for tasks.

web/templates/widgets/todo_widget_tasks.hbs

@@ -8,9 +8,9 @@
             </div>
             <div>
                 {{#if completed}}
-                <strike><strong>{{ task }}</strong>{{#if desc }} - {{ desc }}{{/if}}</strike>
+                <strike><strong>{{ task }}</strong>{{#if desc }}: {{ desc }}{{/if}}</strike>
                 {{else}}
-                <strong>{{ task }}</strong>{{#if desc }} - {{ desc }}{{/if}}
+                <strong>{{ task }}</strong>{{#if desc }}: {{ desc }}{{/if}}
                 {{/if}}
             </div>
         </label>