Moved the ChangedHeaderOrFooter method from SwTextShell to SwWrtShell
for it to be accessible from the SwHeaderFooterWin objects... and that
method didn't really need to be in the root shell.

That fixed a few quirks from the copied code in SwHeaderFooterWin.

Changed the following:
 + Draw a gradient in the separator tabs background
 + Draw a solid line under the dotted one to have a contrast for some page
   background colors.

I'm not a big fan of Hungarian notation, but since we use it, let's
be consistent.

Since doing a hard re-calc can be slow, let's not do this for all
loading test cases, but only when we are testing formula interpretations.

Since each instance refers to a single file format, while its array
instance holds multiple instances of this structure.

we can't assume that always all formulas are recalculated, we might used
cached results