dialogs: use GtkFileChooserNative for remaining uses of GtkFileChooser

- accountcreationwizard, avatarmanipulation, chatview,
  newaccountsettingsview: reformatting and small stylistic changes.

- generalsettingsview, pluginsettingsview, video/video_widget:
  like the above, use GtkFileChooserNative for recent GTK versions and
  fall back onto GtkFileChooser for older versions.

Change-Id: Ic3abcd0f994939a7898c64f17a0f5c1a0c0e46b9