Improve transaction merging

Transaction merging does not provide any visual feedback to the user. This can lead to unwanted results. This mostly happeqns, when the user selected transactions appear in an unexpected order to the matching function and the category assignment disappears for no obvious reason.

The logic has been enhanced to check if an imported transaction which usually does not have a category assigned shall replace an existing transaction that has a category assignment. In this case, the selected transactions are exchanged so that the category assignment does not disappear. In case both transactions have more than one split, the user is asked for confirmation of the operation.

BUG: 435512 FIXED-IN: 5.1.2