Unduplicate file extension lists.

Several different places in the code had lists of recognized file extensions, which is just a problem waiting to happen if/when a new type is added down the line.  This moves most of the file extension checking to ActorUtil and instead has other things use the FileType enum.
Exceptions to this are RageSurfaceLoad and RageSoundReader which have to check the extension to know how to load a file.
This also adds FT_Ini to the FileType enum, and adds webm and wmv to the list of video files to load.