Shows the given element.
Also see up.element.hide() and up.element.toggle().
Limitations
The element is shown by removing the [hidden] attribute set by up.element.hide().
In case the element is hidden by an inline style ([style="display: none"]),
that inline style is also removed.
You may have CSS rules causing the element to remain hidden after calling up.element.show(element).
Unpoly will not handle such cases in order to keep this function performant. As a workaround, you may
manually set element.style.display = 'block'.
