Made AvailableClassesService known to propertyCollection.
Changed variable names in propertyCollection.

Removed ‘$parentSubject’ since it would now be used in only two places
and therefore, in my opinion, just complicates things.

Also renamed ‘$selectedProperties’ to ‘selectedProperties’, since
AngularJS uses the ‘$’ prefix to indicate AngularJS internal stuff, so
no ‘$’ for us.
Moved loading of properties of a certain subject.
Gave 'AvailableProperties' its own factory.

‘AvailableClassesService’ and ‘AvailablePropertiesService’ are now
separate factories in the module 'GSB.services.availableClasses’.
Merge branch 'R1-2' into develop