Adding new non-uniform app icon templates

[ubuntujaggers]

As step one in my icons spring clean, and to help new contributors, I've removed the old squircle template and added one for each of our core icon shapes.

I took the opportunity to do the following:

• Standardise and correct the drop shadow on the full size icon
• Remove unnecessary clipping (we can discuss the policy on clipping in full, but there's no point using it when it isn't needed)
• Other minor improvements like the 16px circle icon, which was a tiny bit wonky.

Here they are:

[Feichtmeier]

Fantastic work Stuart!
Thank you!

[ubuntujaggers]

I think we can make the circle in the smallest size bigger, it's currently 14px to make room for a barely visible 1px drop shadow. The other 16px icons forgo this because space is a premium. I'll update the template and then update all the actual icons.

[ubuntujaggers]

Dunno how to change an existing PR to draft so am changing the title for now...