Add entry under the 'View' menu to hide the toolbar

* feat: JabRef#3 add required functionality to bookCoverFetcher (JabRef#8)

* feat: add a test for requirement RE02 (JabRef#28)

* feat: add a test for requirement RE02

I also had to change the access modifier to `protected` for one download
function. Additionally, I refactored the download method a bit, in order
to mock at test it more easily.

* fix: cleanup imports

* feat: JabRef#4 add background download and refresh when opening preview (JabRef#21)

* feat: JabRef#34 add missing space and fixed style (JabRef#35)

* feat: JabRef#22 add test for requirement RE03 (JabRef#23)

* changed comments to markdown (JabRef#44)

Co-authored-by: Melker Trané <melkert@kth.se>

* feat: JabRef#52 add new test and created setUp and tearDown (JabRef#53)

* feat: JabRef#13 add test for retrieving cover from empty directory (JabRef#51)

* feat: JabRef#54 add more tests and change modifiers (JabRef#57)

* fix: JabRef#43 change variable names to make their type clear (JabRef#59)

* made tests more uniform and made originally private methods private agai (JabRef#62)

Co-authored-by: Melker Trané <melkert@kth.se>

* docs: JabRef#64 added change to changelog (JabRef#66)

* almost done

* fixed comments

* removed line for debugging

* fixed typo

* added static back

* changed !isEmpty to isPresent

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Co-authored-by: SkitArgDuva <117955752+SkitArgDuva@users.noreply.github.com>
Co-authored-by: Melker Trané <melkert@kth.se>