Allow editing imported data before saving

[c0delama]

First of all, thanks for this great piece of software! It is the first financial software that looks promising enough for me, to give it a real serious try to spend a lot of time inputting my historic data.

It would speed up my workflow tremendously if i could edit the resulting ledger file of the import before i save it. No input script is perfect and especially comments about what was bought are impossible to auto-generate. Right now i have to save the incorrect state, switch to the Editor tab, find the ledger file, edit it there and save it again, just to then switch back to the Import tab again.

I see two straight-forward ways to do that:

1. Editing is always possible, but importing a new file overwrites it.
2. Some sort of toggle that switches between "Edit" and "Import" mode on the "Import" tab. While this sounds a bit more cumbersome at first, i consider it a more conservative approach that less likely leads to accidentally overwriting manual input.

Thanks for your consideration!