Save Abstract edits when switching items

Since itemPane.js sets mode before item, the editable-text's value would
get reset before save() ran.

Not touching the header since we're replacing that with an item-box
field (which does not have this bug) soon anyway.

Fixes #3679

chrome/content/zotero/elements/abstractBox.js

@@ -62,6 +62,10 @@
 		}
 
 		set mode(mode) {
+			if (this._mode === mode) {
+				return;
+			}
+			this.blurOpenField();
 			this._mode = mode;
 			this.render();
 		}