Add convenience tasks for building just one buildType

The project on which we are basing this one rarely is built for all three types at the same time. We should provide ourselves some targets similar to the ones the developers on that project have so that we can more easily mimic their real builds.

Start by locating the existing buildType targets and inspect how the three build types are currently selected. Compare them with the corresponding targets in the project being mimicked; the work is done when each build type can be built through its own convenience target.

Written by the indexing model from the issue text.