Improve determination of highest used id

Object IDs in KMyMoney are usually constructed by a letter based lead-in
followed by a numeric value. Adding objects with a different ID layout
could cause trouble when they still contain some numeric values.

The change only takes those IDs into account when determining the
highest used number that follow the above mentioned layout of KMyMoney.

BUG: 397824
FIXED-IN: 5.0.2