Make Find work again.

* glom/appwindow.[h|cc]:
  on_menu_edit_find(): Properly ! the active state.
  set_mode_data(), set_mode_find(): Call the same methods on
  the Frame widget, instead of expecting that to
  happen in a signal handler when we change the Action
  state.
* glom/frame_glom.[h|cc]: Rename, and split,
  on_menu_Edit_Find() to set_mode_find()
  and set_mode_data(), documenting and simplifying
  them a bit.

This needed reworking because Gio::Action does not act like
Gtk::Action. For instance, we must set the state manually,
and changing the state does not cause activate signals to be
emitted.