Problem

Currently there is no easy way to know when the list of selected items in the file browser changes.

Proposed Solution

Add a newselectionChanged signal on theFileBrowser. The args could contain the list of selected items orvoid since they can already be retrieved withbrowser.selectedItems().

Additional context

This would directly help Notebook 7 in the case of adding buttons to the file browser toolbar when multiple items are selected:jupyter/notebook#6888