Handling of predefined cell styles

[JanMarvin]

We might provide functions to modify the set of predefined cell styles. At first glance they appear to consist of a name and xfId pointing to cellXfs. So all the style values have to be available in styles.xml (fills, border, numfmt etc.). Simply adding a builtin style Heading 1 appears not to be sufficient to style a cell. The builtinIds might indicate the position of the cell style in the drop down menu below.

From my understanding who is pointing at whom:cellStyles -> cellXfs -> cellStylesXfs. Not sure why cellXfs is required, if cellStyleXfs basically does the same (handling borders, fonts, fills and numfmts). (This is not true. From [1]: "The xfId [in cellStyles] is a zero-based index, referencing an xf record in the cellStyleXfs collection.")

<cellStyleXfs count="4">
  <xf numFmtId="0" fontId="0" fillId="0" borderId="0"/>
  <xf numFmtId="0" fontId="2" fillId="0" borderId="1" applyNumberFormat="0" applyFill="0" applyAlignment="0" applyProtection="0"/>
  <xf numFmtId="0" fontId="3" fillId="2" borderId="0" applyNumberFormat="0" applyBorder="0" applyAlignment="0" applyProtection="0"/>
  <xf numFmtId="0" fontId="1" fillId="0" borderId="0" applyFont="0"/>
</cellStyleXfs>
<cellXfs count="4">
  <xf numFmtId="0" fontId="0" fillId="0" borderId="0" xfId="0"/>
  <xf numFmtId="0" fontId="3" fillId="2" borderId="0" xfId="2"/>
  <xf numFmtId="0" fontId="2" fillId="0" borderId="1" xfId="1"/>
  <xf numFmtId="0" fontId="1" fillId="0" borderId="0" xfId="3"/>
</cellXfs>
<cellStyles count="4">
  <cellStyle name="Bad" xfId="2" builtinId="27"/>
  <cellStyle name="Heading 1" xfId="1" builtinId="16"/>
  <cellStyle name="Normal" xfId="0" builtinId="0"/>
  <cellStyle name="teststyle" xfId="3" xr:uid="{A2DCCB99-F391-9F4C-91C8-BEB56C8D5650}"/>
</cellStyles>

I've added help wanted, but it is merely an invitation for others to work on this. It should be straight forward, since we provide almost all that is needed. Maybe missing cellXfs and cellStyles in style_mgr, but everything else should already be in place and would simply require some functions wrapping things nicely. Though it requires some further research and testing.

[1] ECMA-376-1:2016: Office Open XML File Formats — Fundamentals and Markup Language Reference

PS: Maybe for a start I would leave out cellXfsand see if that matters after all.

[JanMarvin]

Related: #518