Allow inline calculation of values

[leofah]

When inserting a position or share to a user it would be nice to have an inline calculator. e.g. 3 people bought the same dish for 4,56€ which is then billed as position with 13,68€. For user friendliness it's nice to just insert 3*4,56 and the result is immediately computed.

This will be also useful in many other places, where numbers are inserted, e.g. the value of a share.