add enums to combo boxes, remove 'setComboBox'-Method in MainController because it works only with Strings and again some renaming of old 'degreeModification' to 'coursesOfStudyModification'

# Conflicts:
#	src/main/java/com/example/gui/CourseModificationController.java
#	src/main/java/com/example/gui/CoursesOfStudyModificationController.java
#	src/main/java/com/example/gui/DegreeDetailsController.java
#	src/main/java/com/example/gui/MainController.java
#	src/main/java/com/example/gui/StudentModificationController.java
#	src/main/resources/com/example/gui/course-modification.fxml
#	src/main/resources/com/example/gui/courses-of-study-modification.fxml
#	src/main/resources/com/example/gui/main.fxml
#	src/main/resources/com/example/gui/student-modification.fxml

Gui-V1

See merge request !1

# Conflicts:
#   .gitignore
#   .idea/misc.xml