I added 5 notification icons. They are flat icons that appear a bit bland at
larger sizes, but are nice when smaller like in the Deskbar or in lists.
Here's a PNG of them: https://lut.im/EtRvHPsMNC/T5wXFapUOkej68B9.png
Notify_Notice - your normal info
Notify_Achtung - slightly more interesting than a notice
Notify_Caution - 1st level of escalation, "Odd..."
Notify_Warning - 2nd level of escalation, "Something's not right at all..."
Notify_Danger - 3rd level of escalation, "I smell smoke..."