Make use of mythuiwebbrowser.h bits in mythmusic optional

[ijc]

If qtwebkit (and therefore mythuiwebbrowser) are not available then omit the menu items relating to searching for artwork on the internet.

Fixes #328 (or at least, worksaround I suppose)

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Checklist

• contribution does not duplicate one of our existing pull requests
• contribution is in a branch rebased against master
• code compiles successfully without errors
• code follows the MythTV Coding Standards
• documentation added/updated/removed where necessary
• commits are logically organised and have good commit messages

[ijc]

Looks like I need to rebase onto c37fc10. I'll try and do that over the Easter break, it doesn't look too complex.

(@linuxdude42 FYI)

[ijc]

Rebased.

[linuxdude42]

Thanks.