The goal is preserving the shadow effect with all its attributes using
the shape grab bag. This is the relevant piece of XML in the document:

    <a:effectLst>
      <a:outerShdw blurRad="50800" dist="38100"
      dir="2700000" algn="tl" rotWithShape="0">
        <a:schemeClr val="accent1">
          <a:alpha val="40000" />
        </a:schemeClr>
      </a:outerShdw>
    </a:effectLst>

In first place, we added members to the structure EffectProperties to
store the effect name and attributes. Later, when we create the shape,
we add them to the shape grab bag together with the shadow color (if
it is a theme color we store its name and transformations like in
other cases). Finally, we read back all these data from the shape grab
bag and write them back to the document.

I added a unit test for this shape property.

Change-Id: Idda2d5e2970cb8563e2ed13a84b2fa2d4b99aa70