Add support for monthly investment recommendations

[MadeInPierre]

I'd like to set a monthly investment amount, like 500€, and know how much to invest in each line. This would have two modes:

• Static: No matter the current investment amounts in each line, this mode would give the theoretical amount to invest each month based on the predefined targets.
• Dynamic: This mode uses the current investment amounts to generate a list of transactions to achieve the targets, including 500 "new" euros. This already looks like the current delta feature, but an extension with a fixed monthly income would be great.

[MadeInPierre]

Not really useful actually, does not justify the added complexity, closing.