Add a subscribeSignal method to targets and projects that will make
sure all signals of all project configurations added during the lifetime
of the project/target will get connected (if the type matches).

Use this to connect to some signal in all BuildConfigurations of
a project and get rid of code that keeps connecting to the current
build configuration.

Use Project::buildEnvironmentChanged as an example and convert its
usages.

Change-Id: I689bcebac4b191bf3f8a18765bf18eaac371c5fe
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>