Add file format to filters

Clicking a repository has the following effect:
If the repository that currently is the filter
is clicked, the repository filter will be reset
to allow all repos.
If a different repo than the current filter is
clicked, the clicked repo will become the new filter.
This means that currently either all or exactly one
repo can be allowed in the filter.

All of the above also applies to file format.

The filter for university is not working
since IGitFiles does not have a corresponding field.

The user interface needs to be improved, since it
is not possible to see the current selections.

It might also be beneficial to allow multiple values
for one filter. (e.g. search for files that are either
.md or .java files).