Show cover art in spinny underneath spinny background.

This means spinny backgrounds should be partially transparent so they
can mask the art.
Add optional <ShowCover> bool element to Spinny so skins can disable covers.  (Default to true)

Update LateNight skin with new, better-looking spinnies that show the covers.
Make SelectX elements case-insensitive.
Remove unnecessary .toLower calls

Tweak color of spinnies.
Merge pull request #391 from ywwg/spinny-cover-art

Show cover art in spinny underneath spinny background.
Code fascism.

* Fix compiler warnings in non-3rd-party code.
* Add some missing const-references.
* Eliminate cases of "if(" "( ... )" and "for(".