fix and complete load_file
note: the program now loads default_sale.mmgs on startup.
additionally, the default main grid size (via default_sale.mmgs) has been set to 600x400, based on
defaults in the frontend.
add csapi method to load file
fix small can_apply bug
a failure to initialize every field in an application_node caused it to occasionally take on nonzero values
add call to get_color_of_tile in frontend
give default color a low alpha, to show backdrop picture if available
create type-safe union for stand/stand_template pointers in tiles