Change Percentage to be a full type

The type drn::budgeting::Percentage should move to foundation. It should also behave natively like a normal percentage, i.e. a call of Percentage{20.0} means 20% and all maths operation would then perform the conversions as needed. The precision should not be static and be renamed as an unsigned type. In the UserPreferences file add the support // const QString accountingSurveyWindowPrecisionKey{accountingSurveyWindowKey + "/precision"}.