[core] OSGi packaging

[CrystalMethod]

Currently OSGi packaging is straight forward: each Maven bundle becomes an OSGi bundle by adding the Manifest at build time.

When using XO in the OSGi environment the user only works with the API bundle (+ store provider in some cases) The user doesnt know anything about impl and spi bundle. But both bundles are necessary at runtime: must be part of the runtime. Additionally the impl bundle must be "started" to get the Bundle Listener work properly. Well, one could extend documentation, but I propose to change the OSGi bundle build in a way to make it easier for developers:

a) build a separate bundle that depend on api + spi + impl and contains the BundleListener
b) attach spi + impl to api bundle

[CrystalMethod]

An additional bundle doesn't make much sense here. This new bundle needs access to xo.impl classes that are currently not exported. A new Maven project (OSGi Bundle) with embedded xo.api, xo.spi, xo.impl artifacts seems to fit better.

[CrystalMethod]

There is a "hidden" feature in Eclipse: Bundles with "lazy" activation policy providing SCRs will be started by the framework on startup. I will prepare a PR to make use of this feature and close this issue afterwards.

[CrystalMethod]

should be solved by #169