Emit a signal when the file browser selected items changes

[jtpio]

Problem

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

Proposed Solution

Add a new selectionChanged signal on the FileBrowser. The args could contain the list of selected items or void since they can already be retrieved with browser.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