Recognizing Conjured Items from GMSTs

Resolving https://bugs.openmw.org/issues/1906

It adds a Flag_Bound flag that can be used against an ItemStack's mFlags member to quickly determine whether or not it is a bound item. 

Though the system that assigns the bit (at ItemStack creation) should probably be elsewhere (see: mwbase/world.hpp mwworld/worldimp.cpp /mwworld/worldimp.hpp). I just couldn't find a better place on my own, I tried adding code that'd allow for it to be done against Store <<ESM::GameSetting>> but the use of templates was causing more problems than I was solving going that route.

The code is going this route so that it doesn't have to constantly scan the entirety of the GMST list to determine if an item is bound or not.
I'm not sure why a map is used here. A bool is assigned to the value, but it is never used.
If you don't need the value, then better use an std::set.