WIP: feat(xact): add default time when copying transaction

[Fuco1]

Use the transaction's time as default time when asking when the copy should be created. This allows us to use the relative syntax of org-read-date to move the transaction an <interval> forward or backward from the time of the transaction:

+1d (+ 1 day from TODAY) ++1d (+ 1 day from DEFAULT-TIME, i.e. date of the original transaction)

In other words, single +/- is relative against today and double ++/-- is relative against the DEFAULT-TIME (see C-h f org-read-date).

Note that since currently the DEFAULT-TIME is today, this feature is only strictly adding functionality and should therefore be fully backward-compatible.

[Fuco1]

For some reason the default time is shifted one day back, I think it might be some time-zone related issue with org read date.

[Fuco1]

Oh, this actually changes the behaviour when user only hits RET upon the query. It will copy to the original date instead of today. I think that is a significant breaking change unfortunately :(

I still want to have some option to add past transactions easily, so I'll play with this a little bit more.