Implementing XFExt in .xls import filter

[Callum McGing <callum.mcging@xxxxxxxxx>]

Hello,

In trying to fix bug #142154 I have discovered that the .xls import
filter does not currently read XFExt records (type 0x087D) from MS-XML
2.4.355. This allows for cell foreground, background, and borders to
store explicit colors (see 2.5.155) as well as for background
gradients.

Currently XclCellArea and XclCellBorder only work in terms of palette
indices. Would it be best to extend these to also contain an explicit
Color struct (+ maybe a bool to say if it's paletted or a full color)?
Or would another strategy to implement XFExt importing be preferred?

Thanks

Callum McGing