Select which Semester to download

[N-Coder]

You can select in the dropdown menu which semesters should be displayed, so that you can parse them. But don't forget to revert this setting afterwards.

[fknorr]

The semster is actually already set to the current one upon connecting by using the information from this dropdown, but cannot be set manually from the frontend yet.

[fknorr]

Semester names and IDs are now parsed, but semesters other than the current one cannot be synchronized yet.

[n-st]

My fork has a branch that iterates through all semesters and offers them for download: https://github.com/n-st/studip-client/tree/sync-all-semesters

It does the job for now (assuming you've added a view that avoids collisions between courses that appear in multiple semesters), but it should be improved in terms of efficiency and user interface.

Maybe combine it with #35.