backend_gtk: don't hide FileChooserDialog; closes #1530

Hiding the FileChooserDialog was pointless, because a
new one is created each time the Save Figure button is
clicked in the Toolbar.  Hiding seems to have prevented
the widget from being destroyed based on refcount, and this
led to heavy access to the recently-used.xbel file with
every subsequent keystroke.
backend_gtk3: same fix as for backend_gtk, don't hide file chooser
Merge pull request #2056 from efiring/gtk_filechooser_no_hide

backend_gtk: don't hide FileChooserDialog; closes #1530