gtk: Remove use of deprecated stock items

gtk: Remove use of deprecated stock items

Stock items are deprecated. As are ImageMenuItems. Convert everything to
text only MenuItems, with the same text content as mentioned in the
conversion guide:

https://docs.google.com/spreadsheet/pub?key=0AsPAM3pPwxagdGF4THNMMUpjUW5xMXZfdUNzMXhEa2c&output=html

gtk2 users lose their menu icons as well, but I don't think that's enough
of a problem to warrant keeping around back compat code.

Example error:

ui/gtk.c:1328:5: error: ‘GtkStock’ is deprecated [-Werror=deprecated-declarations]
ui/gtk.c:1335:5: error: ‘gtk_image_menu_item_new_from_stock’ is deprecated (declared at /usr/include/gtk-3.0/gtk/deprecated/gtkimagemenuitem.h:78): Use 'gtk_menu_item_new' instead [-Werror=deprecated-declarations]
     s->zoom_out_item = gtk_image_menu_item_new_from_stock(GTK_STOCK_ZOOM_OUT, NULL);

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>