#377 Clarify wording about metadata in UI

[James SF DeLaHunt]

Test versions and environment
X Lossless Decoder version 20151214 (149.1)
Mac OS X 10.10.5
100% reproducible

To reproduce the enhancement
1. Open XLD... Preferences... observe 3rd and 4th icons along the top. Observed: they are labelled "CDDB" and "Metadata". Desired: they are labelled "Service" and "Metadata", or "Metadata Service" and "Metadata Options".
2. In XLD... Preferences... tab CDDB, at teh bottom, there is a checkbox. Observed checkbox is labelled "Automatically connect to CDDB". Desired checkbox label reads, "Automatically connect to metadata service"
3. Examine XLD menu titles. Look at title of menu between "Edit" and "Profile". Observed: Menu title is "CDDB". Desired: Menu title is "Metadata"

Discussion: The UI is worded as if the name of one metadata service, CDDB, is being used to refer to metadata services in general. Since I never use CDDB and always use MusicBrainz, ths wording sounds odd to me. And, XLD does a good job of supporting a variety of metadata services, and letting the user choose. It seems like the UI wording has not caught up to this situation, however.