Refactor: allow event handlers to have a poll function

Previously only a fixed bounding box could be used.
This was not flexible enough.
T63193 will benefit from this refactor.

Reviewers: brecht, campbellbarton

source/blender/editors/space_script/space_script.c

@@ -116,7 +116,7 @@ static void script_main_region_init(wmWindowManager *wm, ARegion *ar)
 
   /* own keymap */
   keymap = WM_keymap_ensure(wm->defaultconf, "Script", SPACE_SCRIPT, 0);
-  WM_event_add_keymap_handler_bb(&ar->handlers, keymap, &ar->v2d.mask, &ar->winrct);
+  WM_event_add_keymap_handler_v2d_mask(&ar->handlers, keymap);
 }
 
 static void script_main_region_draw(const bContext *C, ARegion *ar)