Added apply_transparency()

[radarhere]

Resolves #6348

Adds apply_transparency(), a method to take a P mode image with a "transparency" key in the im.info dictionary, remove the "transparency" key and apply the transparency to the palette instead.

[hugovk]

Thanks, for a new API, let's add this to docs (an automethod in docs/reference/Image.rst?) and release notes.

[radarhere]

Ok, done

[hugovk]

Thanks!