cost improvements: make cost splitting optional (and just use: overview of costs)

[wivaku]

In general I am interested in cost tracking, but often I am not interested in cost splitting.

I want to know the total costs for budgeting purposes, but when traveling with my partner I don't split the costs.

I could ignore all of the built-in cost splitting features but that means that for each expense I see "unfinished!" warning, or I manually need to set myself as the payer to get rid of the warning (and still: ignore all of the cost splitting details).

[gittiedank]

#1643

[wivaku]

Indeed similar. My request is more basic I think: ability to disable cost splitting per trip. Related: make it a user setting to enable/disable it by default and change it per trip.