Hide metabox or show message when theme has no sidebars

[nickcernis]

On single-column themes where all sidebars have been unregistered, such as Ambiance, Simple Sidebars outputs an empty meta box:

Suggest that we either:

• Prevent meta box output for Genesis child themes with no sidebar layout.
• Display a message in the meta box saying, “Your active theme does not support sidebars.”

[dreamwhisper]

@nathanrice This appears to be a duplicate of #9 but was fixed in 7395bd6

I'm not sure that ever went into a released version, but it is hidden in RM and not in the new or 2.0.4. I saw the commits to the 2.0.3, master and rainmaker-master branches. ¯_(ツ)_/¯