Tab pane, persistence of language: js error

[deining]

As mentioned and discused in #1202, a regression was introduced with 10fae88:
When persisting the language of a tab pane, the following error is now thrown in the browser's javascript console:

Uncaught TypeError: $(...).tab is not a function
    <anonymous> http://localhost:1313/js/tabpane-persist.js:7
    <anonymous> http://localhost:1313/js/tabpane-persist.js:6