using the mouse.

 * gladeui/glade-design-layout.c:
   o fixed offscreen bg rendering bug (style context from the embedded widget should be used)
   o Implemented ACTIVITY_MARGINS
   o include widget margins on selection drawing

 * gladeui/glade-cursor.c:
   o made cursor->selector == NULL
   o Only set cursor if the current cursor is diferent

 * gladeui/glade-project.h: added GLADE_POINTER_MARGIN_MODE

 * gladeui/glade-placeholder.c: fixed function prototypes.