sw: Small code rewrite

The purpose is improve legibility of future patches

Change-Id: I15bcfdc74feab25cf3ce4dcf246969f3d9f152d2
ooxml: preserve font theme color property

The property is called w:themeColor and it belongs to the w:color tag
inside run properties.

On import, the themeColor prop is stored in the character interop
grab bag together with the val prop.

On export, the current color of a text run is compared with the
stored val prop to know if the color has been changed during the
document edition. If it hasn't changed, the themeColor property
must be written to the document; if it has, it must not be written to
avoid overwriting the new color.

Also added a unit test for this property.

Change-Id: Icc95ee023aecc741adcba23d23206aadd6c30e1f
ooxml: preserve font theme color shade property

Change-Id: I40fa3f172f98b319a97a855231b854651be2d784