Made Builder properly reacts to changes of functions paths

* Refactored model plugins checking
* Added check of model before close of Preferences dialog box, user can now cancel close of this box if he doesn't approve proposed changes
* Fixed Builder updates after a change of functions paths (it comes from Preferences dialog box or file monitoring)
Add signature declarations for time scheduling of functions

* Added macros for scheduling declarations
* Updated functions reporting in openfluid program
* Updated signatures of some test functions
* Updated function signature unit test

(closes #76)