Allow the selection of a blank placeholder image

[IBNobody]

We're getting closer @PhilipK !

#105 and #100 have both been addressed, but now a new problem arose.

Clearing / banning the logo for Moonlight works, but because there is no logo for this title, Steam will display the title's text.

What we need is either a blank logo from SteamGridDB or a generated all-alpha png.