XBL: ability for component to export XPath functions

[ebruchez]

This just makes sense as a way to communicate.

<xbl:binding>
    <xf:function>...</xf:function>

The function is accessible:

• from the outside
• possibly from the inside too (why not?)

From the outside, the function can be in a namespace. That could be something like:

xmlns:foo="something-with-id-of-the-control"

or:

xmlns:foo="something-with-id-of-binding"

• if id of control: function doesn't need to take reference to object as parameter
• if id of binding: function needs to take reference to object as parameter

[ebruchez]

This is related to #764. It probably makes a bit more sense to first implement support for functions in models, and then see how that can be extended to XBL?