Activity notifications try 3

CC @manuq

(Pretty much the same as last, but with what @tchx84 added)

`toolkit pr` https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/114

I added the ability for activities to do the notification dance. The ui is just the systems notification ui but merged with the activity palette. I added a activity.notify function in the toolkit. Here is a picture (all else is shared with other notifications):
 
![image](https://f.cloud.github.com/assets/6022042/2429481/825873f4-ac85-11e3-80ba-409598cabb19.png)