make PollHandler an interface

- introduce SimplePollHadler for services having same poll and final result
- introduce AbstractPollHandler and AbstractPollHandlerBase classes for services which need more poll customizations
- removed ugly instanceof operator in former PollHandler class
fix model service

- change updateProjectModel(Project, ModelDiff) to execute MAQL script from given ModelDiff with the least side-effects
- make getProjectModelDiff(Project, DiffRequest) private as it is not usable for the user
- add tests

Closes: #20
showcase