wish: improve handling of end of month scheduled transactions

[Kinnin Vo-Shay]

Thank you for the recent updates to the scheduled transactions UI! They've been great, making them easier to use!

Having started using them more, I found that the "every 1 month" functionality is slightly off when dealing with transactions scheduled for the end of the month, where a shorter month changes things unexpectedly.

As an example, a transaction that recurs every "1 month", scheduled for January 31st, will go as so:
January 31
February 28
March 28
April 28
...
Homebank handles January and February just fine, but notice that the transactions March onward are scheduled for the 28th, due to February's shorter length.

If I schedule something to recur on the 31st, I like the fact that Homebank catches the shorter month and handles it automatically. However, after that short month, I think it should continue to attempt to schedule following recurrences on the 31st.

This is Homebank 5.1.2, on Windows 10.