"List view" of many references in left sidebar: list view with sort, filter, add, export functionality at the top

From #159

This can be implemented like the following image:

• Add and export actions in the panel actions (right icons)
• Sort in a menu triggered by the rightmost icon (3 vertical dots)
• Filter by typing in the text area

@hammer how do you see the export functionality? Should we download a .bib file?