[GLView Drag/Drop] Initial Commit

Issue #35

Currently has rudimentary support for BSShaderTextureSets.  Dragging
diffuse, normal ('_n'), glow ('_g'), cubemap ('_e'), environment ('_m')
are assigned to their proper slots.   The slot numbers are specific to
Skyrim, so only diffuse might work properly in earlier games.

It also attempts to strip absolute paths if the filepath contains
'textures\' and the relative path starting at 'textures\' is found in
the BSA archives or the folder paths in Settings.