Add icons on notification's buttons

Hello,
I notice that you have added buttons on notifications, but there is an issue that some users may confuse "Do not play again" button with "Pause". So I think it's better to add icons on the button to make it more humanized, and I make these commits.
The two icons `img/icon2/Next-notification.png` and `img/icon2/Trash-notification.png` used for button icons were cropped from existing icons for the existing icons have a large padding.
I hope you will like it :smile: 
Thanks