Review Request: Refactor kotext/KoTableCellStyle

[Pierre]

For the record :
1) since these changes imply a lot of tiny changes spread accross 
kotext/textlayout plus a few tiny extra places, I prefer splitting it in several 
parts, hence this first step that only provides cleanups and almost get rid of 
KoTableBorderStyle…
2) the final aim is to be able to implement proper saving support for borders by 
just reusing the KoBorder code that works… Later, other classes like 
KoParagraphStyle shall be converted to using KoBorder too…