get rid of remaining `#imageLiteral`

[r10s]

#imageLiteral(resourceName:) has 16 occurrences in our source while the equivalent UIImage(named:) has 54.

it does not make much sense to mix these two approaches, also there are some disadvantages in #imageLiteral(resourceName:):

• #imageLiteral make it hard in xcode to see what is going on: they are replaced by some icon, so you cannot easily see the name of the image
• searching a bit around, there seem to be more xcode issues with #imageLiteral we did not encounter yet :)

so, to sum up, i do not see a single reason for #imageLiteral, let's get rid of that - also to not manifest things by copy+paste as at #2245