Add initial implementation of Plugin API providing the list of plugins,
with integration test stub.
Since we're not loading plugins during acceptance tests, the API will
always return an empty list, and the test only really serves to ensure
that there are no missing dependency injections etc.
After finding a way to make the list return actual data, either by
installing dummy plugins or mocking the PluginLoader, the tests can
be expanded.
Change-Id: I02e3b34a64fa1134dee4b1375fdb9a635651c0ca
e253101f5a