Allow overriding the 'cost type' on each activity

[LarsKemmann]

If I assign a 'None' or 'Indirect' resource to an activity, I may still want that activity to count towards 'Direct' cost, and vice-versa.

[LarsKemmann]

An example would be assigning the architect (normally an Indirect resource) to a 'Requirements' activity that's part of the earned value of my project (and hence should be a 'Direct' cost).