Create GIMP_CONFIG_DEFAULT_(ICON_)?THEME in gimpguiconfig.h to set
defaults in a single place, except for libgimpwidgets/ which cannot
include from app/.
Add gimp_stock_set_icon_theme() which can be called at any time, also
before gimp_stock_init(), in which case we avoid setting the configured
icon theme twice on startup. Call it from libgimp/gimpui.c and
from app/gui/icon-themes.c so the app and plug-ins use the same
icon theme.
