oox, writerfilter: add a14 namespace and tokens

Added the artistic effects defined in the MS-ODRAWXML extension. Ref:
http://msdn.microsoft.com/en-us/library/dd905216(v=office.12).aspx

Change-Id: I24c5e5bbe569f160d300a7a7293ca047cb7a1b88.
ooxml: preserve artistic effects on shapes.

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
ooxml: Preserve the original picture in artistic effects

When Word applies an artistic effect, it creates two embedded files;
one contains the bitmap with the effect and the other one contains the
original bitmap to be able to undo the effect.

This patch reads the original bitmap, stores it in the shape grab bag
and saves it back to the docx file. Added unit tests too.

TODO: right now, if two effects point to the same original bitmap it
is stored twice, we should improve this.

Change-Id: Ia72034a257739abe4ffafa0f42b2a912e4bf9436
ooxml: Do not repeat wdp files in artistic effects

When two pictures apply different effects to the same picture, it is
only saved once in the original document. Added a cache to DrawingML
to know if the picture has already been exported, and added a test
for it.

Change-Id: Ia25f3d8f2f46d61f18aefc22fdfdbcdc72f2d916