Add possibility to use diffent (PNG) stencils as covers

[padrino-x]

I think 😎, it would be a nice feature if the user could set different stencils, by adding PNGs (using transparency/alpha channel) to the configuration, which then could be used for different cases.
Ideally it would be possible to start/open FSB with a selected pencil by using URI.
This way one could have different types of coverage (complete, partial/selected areas...) for different apps, that would be cool. 😅

[padrino-x]

I think #13 and #54 could be joined ro this request, to clean the issues up a bit... ;)

Or maybe the should be kept separately, as they different. 🤔🤷🏻‍♂️