Make Transform filter accept the file paths of the stylesheet

[furfurylic]

As of now, Transform filter can allow style attribute only to be a URI. This patch relaxes this situation and make it allowed to be a file path too.

This makes

<!-- This property shall be a full file path -->
<property name="styledir" location="style"/>

...

<transform style="${styledir}/style.xsl">
...
</transform>

allowed also in Windows.