Fixes points 3. and 4. of #3621 (comment)

Another issue is that the search-bar is connected to the splitter between main-table and the left side bar. When I resize the main-table, it is possible to move the search-bar over the toolbar icons:

When closing the sidebar, the main-table is not maximized anymore. As can be seen in the image above, we end up with a gray area where the side-bar was.

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)