Create user interface for creation of MasterPlan

The MasterPlan datatype has been defined as the data needed for saving a master plan. A user interface for creating objects of this type with relevant information is needed. This interface should appear after clicking a "Make Plan" button. The name of the master program and the start year should be selectable at the top. The specialization should also be optionally selectable. It should be possible to select which semesters to include by adding them one by one, initially none are selected. Courses should be addable to the semesters, even from other semesters (with the semester field of PlannedCourse set to the semester, else null), via a search/table like menu.