Added `Model` class, generating SQL moved to main script

Intance of class `Model` contains all tables and relations in current model.
Generating SQL reacts on event invoked by main toolbar.
Added `toJSON` methods of table and relation model, added relation model test
Added `toJSON` for Model class and tests for all its methods
Added new toolbar icons, modified CreateRelation tool, added new tool

Added self created toolbar icons. Added parameter identifying to constructor
of CreateRelation tool that defines which tool is default type of relation.
Added new tool to toolbar that creates non-identifying realation in default.
Fixed setting of child table columns when type of relation change
Added blind save-model tool into toolbar
Added route for saving stringified model to filesystem