Use nested categories in BudgetMonitoring

[nuvitong]

The BudgetMonitoring report now uses nested categories, which are stored as adjacency list since MMEX 1.6.2.

• nested transfer categories of any level can be substituted
• nested transfer categories of any level can be re-assigned

Additionally the report now prints the time period in the report header, which is used to selectd the data.

BTW: perhaps it makes sense, to tag the one commit, which contains all reports compatible to the DB schema of version 1.6.1 and below.