Added the LetWidget

The LetWidget is a replacement or enhancement for the set widget.

If it comes to defining more than one variable I find it more than cumbersome to do something like this:

    <$set name="a" value="1">
        <$set name="b" value="2">
            <$set name="c" value="3">
                <$set name="d" value="4">
                    <<someMacro>>
                </$set>
            </$set>
        </$set>
    </$set>

I felt that this level of complexity is not required and so I created the LetWidget:

    <$let a="1" b="2" c="3" d="4">
        <<someMacro>>
    </$let>