fdo#79691: Fix ppt files embedded in .docx documents

Ensure that the proper media type and relation type are written in the
.docx document for a .ppt presentation, because they are different
from the ones for a .pptx file.

Change-Id: Id91269e49c0effb35415ae8827ff949e69e7063f
fdo#79558: Preserve missing attributes in paragraph w:shd

The missing attributes are w:val, w:color, w:themeColor, w:themeShade,
w:themeTint, w:themeFillShade and w:themeFillTint.

Change-Id: I67dd7c367d820216c1d11f82e3430fbf550cede8
fdo#79558: Do not overwrite w:shd value

When a w:shd has some pattern with two colors, LO blends both to
render the paragraph background. We must compare that blended color
with the paragraph color on export to know if the user has edited it
or not. We were using the w:fill attribute to compare, but that was
incorrect.

Modified an existing unit test to check this behaviour. The unit test
had to be retouched because Word remove some redundant information
from the original .docx file when I saved it again with some
background changes.

Change-Id: Ia2f1ddc4afd2637e1d87b6eccd441c26853045c4