Bitmaps can define artistic effects like in the following example:

  <a:blip r:embed="rId5">
    <a:extLst>
      <a:ext uri="{BEBA8EAE-BF5A-486C-A8C5-ECC9F3942E4B}">
        <a14:imgProps
         xmlns:a14="http://schemas.microsoft.com/office/drawing/2010/main">
          <a14:imgLayer r:embed="rId6">
            <a14:imgEffect>
              <a14:artisticMarker trans="14000" size="80" />
            </a14:imgEffect>
          </a14:imgLayer>
        </a14:imgProps>
      </a:ext>
    </a:extLst>
  </a:blip>

LO core doesn't support them, but I'm preserving them using the shape
grab bag. Bitmaps must not be transformed to a SwXTextGraphicObject
so the grab bag of the XShape is not discarded.

Added several Context and Properties objects on the import side to
traverse and save the relevant tags, and added the corresponding code
on the export side to extract the grab bag and output the effect back.

Also added a unit test for a selection of artistic effects.

TODO: Word saves the original bitmap as an embedded wdp file so the
effect can be undone. We must preserve it too and add the reference to
the a14:imgLayer tag.

Change-Id: I61d427f83e4c8f353eb073da0114cd73ba50ba4b