Support XPage Layout inheritance from common configuration

Hi @gijs.hendrix, I kind of regret the choice to not inherit from 'parent configs'. It made sense from SaaS pov to not want this, and it took me quite some code to support this. But then in SaaS, in the end, we just don't inherit from common configs. So the code was ripped out for SaaS again. I think we can better switch the behavior to support inherited xpage layouts just like normal inheritance for PaaS as well. I think we should be able to do this withouth BC issues...WDYT? Support this in 14.4.0 and indicate that indeed we choice was a poor choice? I think we documented this behavior also.