pta-me improvements

[michielbdejong]

We need to get the description for each transaction,
but in the light of #147 it's not automatically clear where to get that from.
The current implementation books everything from assets to income, which is a bit questionable.

Let's start with bank statement imports.
The nice thing is we're already deduplicating movements from/to the user's own accounts (e.g. checking -> savings), so that those don't show up twice.
In pta-me we want to start with a list of 'own' accounts, and then for starters for instance only look at bank statement imports, leave out implied purchase generation, and then have one account for each bank account, and for each outer movement check if it's incoming or outgoing, and just write it down in the journal including its description.