[settings] Promote hideemptyseries to GUI setting.

This promotes `videolibrary.hideemptyseries` from an advanced setting to a GUI
one.  This is enabled by default.

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See https://github.com/xbmc/xbmc/pull/5752#issuecomment-68108700 and below. This is valid even without 5752.

Please note that **default value is changed** from false to true. I'm fine if you want it reverted.