On charts page, only displays the sum of a category

This is partially linked to https://framagit.org/bnjbvr/kresus/issues/435

The problem is similar: with have in and out operations which looks like a lot of money is moving when it isn't. I suggested #435 for internal operations between accounts (so from Livret A to PEL for example), I'd like to do the same for categories. For example, I have a "Loyer" category, where I have a -800 (I'm paying the rent) and a +400 (my roommate is giving me half of the rent). I also start the month by putting money on my Livret A (so I have a -XXX labeled with the "Epargne" category on my main account) but sometimes events occur and I have to put money back, so I have a +YYY also labeled "Epargne", from my Livret A back to my main account.

I suggest to add an option "Sums categories" (which could be set on chart page). So for every category (grouping operations without category together), sum every operations, being + or -. That way, you even become able to display the pie chart when both Income and Expense checkboxes are checked :)

(We can discuss about it in French if that's not clear ;P)