Full-screen browser 2dx_image: Lattice determination

[scherers]

In the fullscreen browser for FFTs -> Lattice Determination: Instead of "Accept as Second Lattice", there could be the following buttons: "Swap first and second lattice" , "Copy first onto second lattice" , "Copy second onto first lattice". This way, the current spot selection scheme and "Accept as first lattice" could remain. The user would always only work on the "first lattice", but could use the second lattice position as safe heaven for a determined lattice, and then try manually to find other lattices. If these turn out to not work, the user could copy back the previous attempt onto the "first lattice" register.