[misc] Remove custom 'withTip' implementation.
XWIKI-8777: Users are allowed to create pages without entering a "Page Name" on IE8/9
* Improve the 'withTip' behavior for browsers that don't support the placeholder attribute:
** Mark the input it's empty
** Allow the user to type exactly the default value (which should be valid)
* Use a custom validation on page creation for the page name input, takeing into account the default value.