...and add some basic tests.

Introduce the abstractions ProjectInterface and ToolChainInterface in
order to break the dependency to the ProjectExplorer. Also, some simple
logic can go there to simplify the (Base)ProjectPartBuilder.

Change-Id: I6c50a1804ce62098b87109931eb171f5c2542937
Reviewed-by: David Schulz <david.schulz@qt.io>