Add borders to collapsed collections/items splitters
Add borders to collapsed collections/items splitters

We can think about making them more wider, etc., but this makes them ever so
slightly more visible.

sendecomp/zotero,hoehnp/zotero,hoehnp/zotero,egh/zotero,robamler/zotero,retorquere/zotero,sendecomp/zotero,LinuxMercedes/zotero,retorquere/zotero,adunning/zotero,sendecomp/zotero,egh/zotero,adunning/zotero,retorquere/zotero,egh/zotero,gracile-fr/zotero,robamler/zotero,robamler/zotero,LinuxMercedes/zotero,gracile-fr/zotero,hoehnp/zotero,rmzelle/zotero,adunning/zotero,LinuxMercedes/zotero,rmzelle/zotero,gracile-fr/zotero,rmzelle/zotero