This is an attempt to complete the proposal T53798.

Since the preview of area split has to share internal variables of SCREEN_OT_area_split, I decided to convert the drawing function in ED_screen_draw into a callback.

But as the existing draw_callbacks are limited to areas. The solution taken was to extend the API including draw_callbacks to the struct wmWindow.

So this patch can be divided into three stages:

• Create API for draw_callbacks in struct wmWindow;
• Split the ED_screen_draw function into ED_screen_draw_edges, ED_screen_draw_join_shape and ED_screen_draw_split_preview and use the last two functions as callbacks called in SCREEN_OT_area_split and SCREEN_OT_area_join respectively;
• Calculate a snap factor in ED_screen_draw_split_preview.

I will gladly accept other suggestions ;)