Support price validation

[TheKeymaster]

libflexport allows any type of string (except it is empty) to be put in the XML. This could result in an validation issue, because the XML Scheme only allows a decimal.
The export allows following:

cvc-datatype-valid.1.2.1: 'blubbergurken' is not a valid value for 'decimal'.

A potential fix would be to have an own price validation that checks for is_float().