Use uniform capitalization in VideoConfigDiag

Nouns should be written capitalized for consistency.

Concerns have been raised on IRC about how sensible this scheme of capitalization is, but that's not what this PR addresses. If someone wants to change the scheme of capitalization, then this should be discussed in a separate PR. For now I want to restore full consistency of the scheme which has been in use since before Dolphin 3.0, though.