Preferences: remove "multi page handling" preference

Proper multipage/frame export support is a ways down the road, so this
is extremely hackish right now - but basically, PD will always load
multipage images (TIFFs, icons) and multiframe images (animated GIFs)
using the "load each page to its own layer" system.  At present, there
is regrettably no way to save such files - they will always be flattened
prior to saving.

I hope to address this more intelligently in 6.6.