Update menu headers and main menu rows

Modified all menu headers to be in ALL CAPS, made memory-saving
adjustments to "main_menu_draw_row_callback",
"pebble_options_menu_draw_header_callback", and
"heavy_items_menu_draw_header_callback", and replaced
MENU_HEADER_STR_LEN with HEAVY_ITEMS_MENU_HEADER_STR_LEN as the heavy
items menu is now the only one that requires a STR_LEN constant.