*Title: Hide (unpublish) feature

Incentive:

Unpublish is standard functionality and is very much requested by the
community.

Since publishing means adding something to the content tree unpublish
implies removal of something from the content tree. This is not the case
with the solution being described and therefore the feature is called
"Hiding" instead.

*Documentation:

The feature lets user temporary hide a node or subtree from the conten tree,
with the ability to show it again later at the previous location.

Eeach node now has two extra boolean attributes:
H - hidden;
X - invisible.

The Hidden (H) attribute is set by user when he wants to hide a
particular node. A node that is hidden will immediately be marked "X"
for invisible, and all the node's descendants are recursively marked
with "X" as well.

So, the new attributes mean the following:
- HX: node has been hidden by a user and it is invisible;
- X : node is invisible because its ancestor is hidden.

General notes:

- A node cannot be(come) visible if its parent is not visible.
- If a user tries to hide an already invisible node then the node is just
  marked as also hidden (in addition to invisibility).
- If a node is hidden and its parent becomes visible, the node remains
  hidden, and its descendants remain invisible.
- NB: In order to disallow user from seeing invisible nodes in a given
  siteaccess you should explicitly set the appropriate INI setting (see
  below).

Look at the following figures to understand the hiding algorithm better.
On these figures:
N - visible node
H - hidden node (which is also invisible, of course);
X - invisible node (because of a hidden ancestor).
Node being processed is marked with an arrow.

So, there are four possible cases:

1. hiding a visible node:
content tree before:
                 N
                / \
            -->N   N
              / \
             N   N
            / \
           H   N
           |
           X

content tree after:
                 N
                / \
            -->H   N
              / \
             X   X
            / \
           H   X
           |
           X

2. hiding an invisible node
content tree before:
                 N
                / \
               H   N
             /  \
        --> X    X
           / \
          H   X
          | 
          X

content tree after:
                 N
                / \
               H   N
             /  \
        --> H    X
           / \
          H   X
          | 
          X
3. unhiding a node having visible ancestor:
content tree before:
                 N
                / \
         -->   H   N
              /     \
             X       X
            / \
           H   X
           | 
           X

content tree after:
                 N
                / \
         -->   N   N
              /     \
             N       X
            / \
           H   N
           | 
           X

4. unhiding a node having invisible ancestor:
content tree before:
                 N
                / \
               H   N
              / \
             X   X
            / \
       --> H   X
           | 
           X

content tree after:
                 N
                / \
               H   N
              / \
             X   X
            / \
       --> X   X
           | 
           X

Required privileges:
To (un)hide a node the 'content/hide' permission is required.

New INI settings:
site.ini:
 - SiteAccessSettings/ShowHiddenNodes (true/false):
   Determines whether to show hidden nodes in this siteaccess.
   The default value is true.

New DB fields:
 - ezcontentobject_tree.hidden
 - ezcontentobject_tree.invisible

New parameter for template function fetch():
- fetch( 'content', 'list|list_count|tree|tree_count', ... )
  now accepts new boolean parameter 'ignore_visibility'.
  This might be useful to fetch/count invisible nodes disregarding the ini setting.

New access function:
 - content/hide. Note that it is possible to limit (un)hide access only to a subtree.