Add support for parametrized values into GSFProperty assets

[fvillalba]

It would be VERY useful (and reusable) if we could parametrize the value of a GSF Property.
That would make it very simple to use GSFProperty assets as a representation of labels / error messages.
One could do things like:
< gsf : property name="internal_label_Ndocsfound" var="putMyParsedLabelHere" >
< gsf : parameter name="numdocs" value="<%=results.getSize()%> / >
< / gsf : property>
... and define the value of the GSFProperty asset as:
Found Variables.numdocs matches.
OR
Found $$$numdocs$$$ matches.
OR EVEN
Found ${numdocs} matches.
... which would produce at runtime:
Found 24 matches.