Support Ignored flag

OpenMW doesn't respect the Ignored flag for records. components/esm4/common.hpp defines the constant (though I can't find any usages) but it appears to be entirely absent in the esm3 code.

One potential/likely side effect of this is that mods containing dirty GMST records cleaned by marking them as ignored (using TESCS) aren't clean when loaded in OpenMW but are in Morrowind.exe.