Creation of empty ProjectChecker

* Deactivated BuilderModel checking

(references #96)
Implemented ProjectChecker

* Gathered all checks (from Engine::pretestConsistency and BuilderPretestInfo) into ProjectChecker
* Improved check of generators consistency

(references #96)
Add check of monitoring

(references #96)
Reactivation of Model checking

* Moved check of function plugins presence from BuilderModel constructor to EngineProject constructor
* Cleaned code

(references #96)
Add check of units relations at BuilderDomain creation

(references #96)
Add a reverse orientation method for LineStringEntity

* Added About dialog window
Merge branch 'linestringgraph-reverse' of https://github.com/rabotin/openfluid into rabotin-linestringgraph-reverse