lv_style_register_prop() function

a new property ID, or LV_STYLE_PROP_INV if there are no more available. @example lv_style_prop_t MY_PROP; static inline void lv_style_set_my_prop(lv_style_t * style, lv_color_t value) { lv_style_value_t v = {.color = value}; lv_style_set_prop(style, MY_PROP, v); }