zoom-focus better

* app/display/display-enums.h: Added
GIMP_ZOOM_FOCUS_RETAIN_CENTERING_ELSE_BEST_GUESS.

* app/display/gimpdisplayshell-scale.c
(gimp_display_shell_scale_get_zoom_focus): Take the new enum into
account; if the image is centered, keep it centered, else use the
best-guess method.

* app/actions/view-commands.c (view_zoom_explicit_cmd_callback):
Use the new enum for explicit zooming.

* app/display/display-enums.c: Regenerated.

svn path=/trunk/; revision=27653