Fix applying “standalone” display mode

[lil-greasy]

Considering the way PWAsForFirefox is structured, I realize this may not be possible without major not-bloody-likely changes to Firefox itself. But it would be great if PWAsForFirefox could implement the “display: standalone” manifest setting.

[filips123]

This is already supported, but it broke in 2.13.2 because of some other fix. I will fix it soon.