Rename water_nm file and move it to the vfs

!2641 (closed) adds a new file "omw_ripple_blob" for water ripples simulation (no longer a thing now), to stay consistent and to avoid conflicts with mods; would be nice to rename the file "water_nm" to "omw_water_nm" and move it to the same place as "omw_ripple_blob" and the UI textures:

"files/data/textures/"

Note: There are mods that add files with names such as "water_nrm" so it's not that hard to also have a file conflicting with OpenMW's was the reasoning for the rename.