Add support for default account pictures

### Problem

Elementary OS does provide any default images to choose from when setting the user account picture.

The only option you have is to use your own image:

I am personally not keen on seeing my own photo as an account picture, and so it would be more convenient to just pick one from some defaults. For example, elementary does a very nice job with the default wallpapers which I rather like.

I also use multiple user accounts locally, e.g. a work account, a home account, another account for doing demos, and so having different account pictures for these accounts is welcome.

Being able to chose from some default images was always something I liked on both Windows and Mac.

### Proposal

Include default account photos for the user to chose from, similar to the Windows and Mac screenshots above.

### Prior Art (Optional)

Mac OS provides default account pictures (screenshot taken from the web but something similar is in mac OS 12):
Windows has also done a similar thing: