review and consolidate event/state constants used in the ntrack common/backend/glib/qt layers

currently the constants used on the various levels feels redundant.

The initial idea was to make the comon-backend layout purely emitting events about state changes and have the common layer aggregate that and deliver higher level events and aggregated states.

also glib-abstraction-layer currently has its own constants and provides each own not-really-redefined abstraction on top of the already aggregated common layer, which feels wrong and needs to be consolidated - most likely by exporting ntrack-constants.h as a header and allowing all toolkit wrappers to refer to that directly.