Categories as a root of other objects

[arkadius]

Now categories are assigned to each objects (sources, sinks, services, custom nodes, globalVariables etc.) in relation one to many. This design choice causes that some configurations are hard to achieve e.g.:

• For one category you can't have global variable with other fields than in other one
• If you want to have object executed in runtime differently according to category, you need to create many such objects with other ids (see #122)

I think that api should be redesigned that categories should be a root objects for others. Process in runtime should have information of category and should create extensions based on it.