app-window: Move show_properties() from App

show_properties () only uses things that are part of AppWindow ('view',
'current-item') or will be moved to it in next patches ('selection-mode',
'ui-state') and so it should be moved to AppWindow.

This is needed to drop the use of AppWindow singleton and therefore to
make multiple windows possible.

https://bugzilla.gnome.org/show_bug.cgi?id=732098