Add transferred/received amounts in statistics page

[zorun]

[zorun]

Names are temporary, see discussion in #1299

I have tried several orders for the column, this one seems clearer to me, but I'm intestered in feedback.

The negative signs are intended to show that all columns should sum to the balance.

[zorun]

Some ideas for improving the rendering after showing this to somebody:

• there should be two clearly separated sets of columns:
  • me and the group: how much did I spend for the group, how much did the group spent for me
    • one column with "payment/contribution" (currently called "Paid" in my screenshot above)
    • one column with "expenses involving me" (currently called "Expenses" in my screenshot above)
  • money transfers: individual transfers of money between participants to restore the balance of the group (partially or completely)
    • "transfers"
    • "received transfers"
• the two sets should be easy to understand, with good names and maybe an icon
• if it's clear enough, there might not be any need for negative signs anywhere