Rename the global variables default_* to option_*
Move extern declarations into local sope in src/gui.c
Return display_help as an output parameter from parse_options()
Use accessor function instead of widget->window (deprecated in GTK+ 3)

Also applies to `widget->allocation.width` and `widget->allocation.height`,
which should be replaced with `gtk_widget_get_allocated_width(widget)` and
`gtk_widget_get_allocated_height(widget)`. But these functions aren't
available in GTK+ 2.x.
Be more specific about void arguments by changing foo() to foo(void)
Rename constants for piece radius and piece border linewidth