[Feature Request] Add alternate mix merge

The app is missing the alternate mix merge capability.
You could call it a round robin style merge in which the tool takes a single page from an input PDF before moving to the next input file. This continue in a cyclic order until no pages are left to copy in all the input PDFs.

This is useful for users who scan dual-sided documents with a single-side automatic feeder scanner which is quite common in budget AIO/multi-function printers.

In the above use-case the user will only add two input PDFs but I don't think the number of input files should be restricted.

The user might have one scan in a reverse page-order so the app should make it possible to choose the page-order of each input PDF.