RFC: implement a page size ksaneoption
This implements a PageSizeOption as a KSaneOption that allows to select e.g. A4, B4 etc. page sizes if possible and sets the TopLeft and BottomRight coordinates for the scanning area accordingly.
As KSaneWidget implements its own sizing options with multi-select features (which imho is very specific to Skanlite), it is immediately hidden there similar to the TopLeft and BottomRight options. It can currently be tested only with the exported list options branch, again here at https://invent.kde.org/astippich/libksane/-/tree/exportOptions and the corresponding Skanpage implementation at https://invent.kde.org/astippich/skanpage/-/tree/optionsModel
It can be argued that such an option should be implemented in the application instead of the ksane library. In my opinion, it is handy to have such an option in the library for the simple use case, as directly setting the TopLeft and BottomRight coordinates is very inconvenient in most cases.