Set relative heights for all default heading styles

The relative height "100%" (used only by Heading 3 now) is converted
silently to absolute height (btw, also in the dialogs and in OOo, too).
Using 101% instead of 100% fixes this problem, so setting a new height
in the parent style "Heading" (for example, by Tools->Options->
LibreOffice Writer->Basic fonts (western)->Heading->Size) can change
the heights of all default headings, including Heading 3.

Change-Id: Ia44ac9d24041b75f0aedd7b56340dd41caf0020e