* app/widgets/widgets-enums.h: Moved the GimpZoomType enum from	here...

* libgimpwidgets/gimpwidgetsenums.h: ...to here.

* app/widgets/widgets-enums.c
* libgimpwidgets/gimpwidgetsenums.c: regenerated.

* app/display/gimpdisplayshell-scale.[ch]: removed
  gimp_display_shell_scale_zoom_step and
  gimp_display_shell_scale_get_fraction from here...

* libgimpwidgets/gimpzoommodel.[ch]: ... to here so we can use these
  utility functions in plug-ins and in the core.
  Also removed the step-size property since the zoom-model now use
  gimp_zoom_model_zoom_step.

* app/actions/view-commands.c
* app/display/gimpdisplayshell-title.c
* app/display/gimpdisplayshell.c
* app/tools/gimpmagnifytool.c: modified accordingly.

* libgimp/gimpzoompreview.c: don't pass any argument to the
  gimp_zoom_model_new function.

* libgimpwidgets/gimpwidgets.def: added gimp_zoom_model_zoom_step
  (gimp_zoom_model_get_fraction was already there)

* devel-docs/app/app-sections.txt: removed
  gimp_display_shell_scale_zoom_step and
  gimp_display_shell_scale_get_fraction.