helpers used in widget implementation 

/* Create a new widget w from an user allocation or by caling calloc() */
#define mbtk_create_wt(widget, type_kw, imps, caller_alloced_) \
  do { \
    if (caller_alloced != NULL) { \
      widget = caller_alloced_; \
      memset(widget, 0, sizeof(*widget)); \
      widget->gt;w.flags.caller_alloced = 1; \
    } \
    else \
      widget = calloc(sizeof(*widget), 1); \
    widget->gt;w.struct_size = sizeof(*widget); \
    widget->gt;w.type = type_kw; \
    widget->gt;w.hvbox.type = HVBOX_WIDGET; \
    widget->gt;w.imp = imps; \
    widget->gt;w.flags.dirty = 1; \
    widget->gt;w.flags.created = 1; \
    widget->gt;w.hvbox.align = mbtk_default_align; \
    widget->gt;w.hvbox.span = mbtk_default_span; \
    widget->gt;w.hvbox.spacing = mbtk_default_spacing; \
    widget->gt;w.hvbox.padding = mbtk_default_padding; \
  } while(0)

void mbtk_widget_minsize(mbtk_widget_t *widget, long *mx, long *my);


/* Call this if previously floating widget got embedded under a parent */
MBTK_INLINE void mbtk_widget_embedded(mbtk_widget_t *parent, mbtk_widget_t *w);

/* Call this to unlink an embedded widget; calls hvbox unlink and unassigns */
MBTK_INLINE void mbtk_widget_unembed(mbtk_widget_t *w);


/* Look up color by kw_clr in w's style and set drawing color for w->gt;win if
   found. Returns 0 on success, -1 if not found */
MBTK_INLINE int mbtk_widget_draw_set_style_color(mbtk_widget_t *w, mbtk_kw_t kw_clr);