Improved custom attributes

*Title: Improved custom attributes.

*Documentation:

In the past only some tags, like 'embed' could have custom attributes.
Now every ezxmltext tag can have custom attributes, and the list of available
custom attributes is set with content.ini setting:

content.ini [tagname].CustomAttributes

This is an array that contains attributes names. For custom tags 'tagname' is
a name of the tag, for example 'factbox'.

To access custom attribute's values in tag's template, use variable that have
the same name.

*Important:

You can get warning when you try to edit existing objects containing tags with
custom attributes if they are not added to the list. To add all existing
attributes to content.ini automatically run the following script:

update/common/scripts/3.9/correctxmltext.php


*Example:

If you want to add 'summary' attribute to the 'table' element,
add the following setting to content.ini.append:

[table]
CustomAttributes[]
CustomAttributes[]=summary


Now you can enter in the XML text block input field:

<table summary='123>
...


In the template table.tpl:

summary:{$summary}


*Example for a custom tag:

[factbox]
CustomAttributes[]
CustomAttributes[]=summary