Contextually-aware inline messages on controls

[frankiejarrett]

We could help users a lot by spotting known conflicts or contextual issues and displaying an indicator beside the Customizer control that is affected.

For example, if a Header Image has been set, then changing the Header Background Color control will have no effect in the live preview pane.

A novice user could potentially change this color, wonder why nothing is happening, and only conclude that their site must be broken. To improve this experience we could simply add a special alert icon next to this control, and perhaps a short message to reveal/explain this conflict to the user.

[aledesma-godaddy]

Closing issues and PRs in preparation for repository archival