Currently one can only choose to include all fragments or only non-test
fragments. But we can already distinguish several other types of
fragments, e.g. those that belong to extensible API or provide native
code specialization.

This now enhances DependencyManager to support a new type
(INCLUDE_PLATFORM_FRAGMENTS) and to make configurable the inclusion of
extensible API (INCLUDE_EXTENSIBLE_FRAGMENTS). Also the
BundleLauncherHelper was adjusted to be more uniform when handling
plugin versus feature based launches reading and preparing for further
customization through launch configs.