Add critter icons for each collection

[grantfitzsimmons]

It would be a nice touch to allow the user to set, at the collection level, an icon that represents that collection so that it can be visually distinguished.

Requested by:
@tlammer, similar to how Specify 6 implemented it

[bronwyncombs]

+1 to this

i would appreciate being able to pick a collection critter that goes by the name in nav menu and selection dialog