Adding type column and filter to assets list

@andy-armstrong - please take a look at this. (Sorry it took so long, other projects took precedence - as you can see I handed it off to @dino-cikatic midway).

This is the first time either of us has made changes to studio code, so hopefully we have everything we need in place for this.

(Also, we have one change from @frrrances herein for testing purposes - a necessary UI change otherwise the implementation looks to complex)
I think (although I could be wrong) that you could just do:

    if requested_filter:

Both None and empty lists are falsy:

https://docs.python.org/release/2.5.2/lib/truth.html