Change dw_notification_new() to take an image path instead of an in-memory HPIXMAP. After examining all the platforms, several require the image to be on disk. It is easier to specify the path and load it on platforms requiring it in memory than to save it to disk on platforms that require it on disk. Currently it does not automatically pick an extension like some other functions, may need to add that feature here too soon. Only tested on Windows in this commit.