Minus key for non-time-duration inputs

[albertheres]

First of all I really like this multifunctional tracking app!

I would like have a minus key available on the numeric keyboard for non-time-duration-inputs. That would be very usefull for adding negative values. Right now I using copy-paste to add a minus sign.

Right now this minus key is only available for time-duration-inputs.

Related issue: #44