Better and more configurable selection of tracks to transcode

FEATURES:
 * Allow to transcode only certain (different format, playability) tracks when
   transferring them to a collection; patch by Anmol Ahuja.

Added 3 checkbox in the TranscodingAssistantDialog:
"Transcode all tracks" - transcode all tracks, the current default behavior
"Ignore files which're already the selected format" - transcode only if source and destination file formats are different
"Transcode only when needed for playability" - transcode only when needed for playability in the destination collection

Anmol, I've made some very minor modifications, compare the diffs if you're
really curious. Thanks again for implementing this, it makes Amarok better.

FEATURE: 312407
FIXED-IN: 2.8
REVIEW: 109781
CCMAIL: Anmol Ahuja <darthcodus@gmail.com>
DIGEST: Amarok can now be configured not to transcode from e.g. MP3 to
MP3 and more. Patch by a newcomer Anmol Ahuja.