filechooserbutton: Emit 'selection-changed' when changing the selection programmatically

filechooserbutton: Emit 'selection-changed' when changing the selection programmatically

We only emitted that signal when the selection changed through the underlying GtkFileChooserDialog.

To do this when the dialog is not active and the selection is changed by the calling program
(instead of by the user), we need to wait until the GtkFileChooserButton's UI has been updated
via an async callback from GIO.  So, we keep track of whether an entry point into the
button's API caused a programmatic change in the selection.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>