Unify Import/Export of Mipmaps.

[Deledrius]

Adds an improved, unified interface for importing/exporting plMipmaps to and from supported formats.

Depends on #44 and H-uru/libhsplasma#255

[Deledrius]

Up for discussion on this is:

• This removes reading/writing of JPEG Alpha files. Are these still necessary now that we easily read/write formats that support alpha natively?
• This reads all images into an uncompressed image data buffer and stores them this way. Would it be preferred to read them directly into the same image compression as the file itself when possible?