Gocept-selenium

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- Update tests to `gocept.httpserverlayer >= 1.4`.

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- Add documentation for the Jenkins integration of screenshots made from
test failures. (13936)

- Webdriver: Add a loop with time-out to ``click`` in order to deal with
``StaleElementReferenceException`` and ``NoSuchElementException``.

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- Webdriver: ``waitFor`` retries an assertion when ``NoSuchElementException``
was raised. (This is useful for assertions like ``waitForVisible``.)

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- Add ``clear`` to webdriver to delete the contents of an input
field.

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- Write junit annotations when a screenshot was taken for assertions beside
``assertScreenshot()``. (13678)

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- Added ``getCssCount`` and ``getXpathCount``, so tests can get a baseline
before an action.

- Fix ``getSelectedValue`` for webdriver.