This fixes bug #65198

* app/core/Makefile.am
* app/core/core-types.h:
* app/core/gimpgrid.[ch]: added new class GimpGrid.

* app/core/core-enums.[ch]: added new enum GimpGridType.

* app/core/gimpimage-guides.[ch]: removed the gimp_image_snap_*()
functions...

* app/core/gimpimage-snap.[ch]: ...and added them here since they
are no longer guide specific.

* app/core/gimpimage-undo-push.[ch]: added
gimp_image_undo_push_image_grid()

* app/display/gimpdisplayshell-handlers.c:
* app/core/gimpimage.[ch]: added grid member to _GimpImage. Added
new signal "grid_changed", added gimp_image_grid_changed(),
gimp_image_get_grid() and gimp_image_set_grid().

* app/display/gimpdisplayshell-appearance.[ch]: added
gimp_display_shell_set_show_grid(),
gimp_display_shell_get_show_grid(),
gimp_display_shell_set_snap_to_grid() and
gimp_display_shell_get_snap_to_grid().

* app/display/gimpdisplayshell-callbacks.c: added call to
gimp_display_shell_draw_grid()

* app/display/gimpdisplayshell.[ch]: added grid member to
_GimpDisplayShellVisibility, added snap_to_grid and grid_dialog
members to _GimpDisplayShell, added
gimp_display_shell_draw_grid(), modified
gimp_display_shell_snap_coords() to use the new
gimp_image_snap_*() functions.

* app/gui/image-menu.c: added grid entries to
image_menu_entries[].

* app/gui/view-commands.[ch]: added
view_configure_grid_cmd_callback(),
view_toggle_grid_cmd_callback() and
view_snap_to_grid_cmd_callback().

* app/gui/Makefile.am
* app/gui/grid-dialog.[ch]: added a grid dialog.