[File Mgmt] Improve API for loading/saving files

Rename and rearrange some functions.  Create public methods for certain
functions.
[File Mgmt] File extension improvements

Made a map to store the filetypes and file extensions, created static
helpers.  Added BTO/BTR to list of supported extensions.
[File Mgmt] Temp workaround for BTO/BTR texture paths

BTO/BTR files would not load their textures because the these files
actually have Data\ prepended to their texture paths, unlike all other
NIFs.
[File Mgmt] Save As dialog selects appropriate filter

The file filter would always default to NIF even if you were Saving As
for a BTR file.  This did not affect the saving of said BTR file, but if
you're going to select another file to overwrite you probably want to
see the files of the same type.