Support re-ordering signal columns

[CrossBound]

Currently when editing a signal you can only add/remove a column, but you cannot re-order them. This means that if you need to change the arrangement you have to remove them all and re-add them. The problem is that when re-adding them you have to find an event with the property and add it from the event.

It would be nice if you could simply re-arrange their order without adding/removing columns.

[nblumhardt]

Thanks for the suggestion, we'll give this one some thought 👍