files-view: Update NautilusPreviewer on slot changed

Nautilus sends updates to previewer when there's changes to the view's selection. This works in all cases, except context switches where the view's selection hasn't actually changed. When we are switching tabs, although the selection for the view hasn't changed, the selection in terms of how the user perceives it has changed.

I'd argue that the logic is slightly more complicated when switching between two windows. For example two windows could be side by side, in which case the user won't want to switch the previewer, unless specifically selecting a new file in the other window).

Send previewer updates when switching tabs (slots)