This was an edit through the github interface which changed more
than intented.

This reverts commit 1720a8e9.

Behavioural changes:
  * buttons
    * no 'do=show' but clean URLs (show and back buttons)
    * the alias 'subscription' for subscribe buttons is now available
  * links
    * draft link has class 'draft', not 'edit'
    * revisions link has class 'revs', not 'revisions'
  * both
    * 'revision' is added as an alias for 'history'

These changes were mainly introduced to reduce the differences between
links and buttons.

Since an unchecked HTML checkbox has no value at all, a hidden field may be
used to specify an off value for the checkbox.

Since the specs aren't 100% clear, dates might be passed in different
formats by various XMLRPC clients. This patch makes date parsing a bit
more flexible.

Unit tests included.

according to the specs a value without a type is
recognised as string. This patch handles empty value
tags the right way as empty String.

This patch fixes a bug introduced in »code cleanup« #0b17fdc6