UI: support for popups starting with buttons activated

While this could already be done from C,
this is now exposed to Python

source/blender/windowmanager/intern/wm_operators.c

@@ -1154,6 +1154,8 @@ static uiBlock *wm_block_dialog_create(bContext *C, ARegion *ar, void *userData)
 	/* center around the mouse */
 	UI_block_bounds_set_popup(block, 4, (const int[2]){data->width / -2, data->height / 2});
 
+	UI_block_active_only_flagged_buttons(C, ar, block);
+
 	return block;
 }
 
@@ -1179,6 +1181,8 @@ static uiBlock *wm_operator_ui_create(bContext *C, ARegion *ar, void *userData)
 
 	UI_block_bounds_set_popup(block, 4, NULL);
 
+	UI_block_active_only_flagged_buttons(C, ar, block);
+
 	return block;
 }