Switch from UIImage(named: ) to UIImage(contentsOfFile:)

[JanNash]

Description copied from the similar resmio-Tables ticket:

As it is explained here and here,
UIImage(named:) caches the imageData and does seemingly not react to memory warnings.

We should test if that's still the case in iOS 9/10 and if so, switch over to using UIImage(contentsOfFile:) (where we should also test for its memory management)

We should also implement our own caching AND (most importantly) a method to purge the cache on memory warnings, maybe even listen to memory warnings internally and manage the cache flushing internally, too.

[JanNash]

Check if this is still valid.