Revamp the merge-system

Merging things from one showfile into another always requires me to open the showfile I want to merge from, go through the different lists and write down what I want to merge. Then go back to the current showfile and check if the target-spots are availiable or if I have to move anything around. Then merge.

It would be great if we could get a system similar to Cobalt, where you can see a list of what's in the current showfile and a list of what's in the showfile to be merged from, side by side.