[Scenario] Separate static costs from cost functions

Draw a distinction between costs and cost
functions. This is to help distinguish
between costs from Activity Types and costs
from sub-activities, to support correct
overlay for MCT-Plugins#119
[Scenario] Implement stub in Activity's cost wrapper
[Scenario] Use label for immutable costs in Info View

Use a label (instead of a text field) for
immutable costs in the Info View, since these
cannot be directly edited.
[Scenario] Sum costs from multiple activity types

When reporting "internal costs" for an Activity
(e.g. for display in Info View or Timeline Overlay)
sum the costs from all Activity Types.
Merge pull request #121 from VWoeltjen/mctPlugins119

[Scenario] Distinguish activity costs inherited from Activity Types