make it possible to hide or remove a category

[ghost]

In MoneyMoney, if you have more than one account linked, it's easier to categorize transactions from account A to B and mark them as transfers. See the official recommendation (in German).

This helps clean up statistics and pie charts in MoneyMoney and won't interfere while budgeting.

[Xiphe]

I've split out collapsing of categories (#40) which might be enough for most use-cases.

But Still want to add a setting to completely ignore a category.