Add merge support to GUI app

[Shrinks99]

Really impressed with the work put into Fluxengine so far! Reading through the documentation, the merge command seems quite useful, but unfortunately I've had a little trouble building the CLI utility on macOS. Perhaps it could be added to the read disk UI to visualize a before and after comparison of merged VS original sectors from each flux file? Maybe this is what the "Go again" button performs, though as far as I can tell that doesn't actually merge the results? A tooltip on that button would also be helpful to clarify its function.

EDIT: After a few more goes, this is obviously not what the "go again" button does! Tooltip would still probably be ideal though :)