Refactor "smart form descriptions" (via tooltips)

Problem/Motivation

The simple/short/tooltip descriptions need some love to deal with recent changes to markup/features.

Proposed resolution

Standardize the feature so it's more stable and displays properly.

Remaining tasks

• Standardize on an actual name for this "feature".
  Done - Now called "Smart form descriptions".
• Only show tooltips for input elements (i.e. not fieldsets).
  Done - By default this only applies to elements which have #input => TRUE set on them. Optionally, one can programmatically specify #smart_description => TRUE to force a non-input element to be processed.
• Tooltips are duplicated on checkbox and radio element types.
  Done - #2461717: Checkbox and radio input elements should be rendered inside label fixed this.
• Tooltips need to be on the child elements for checkboxes and radios element types.
  No longer necessary
• Tooltips should just check for links in the description (<a> elements) to determine "short description" eligibility.
  Done - There is now an "allowed tags" setting with sensible defaults that assists in determining this filtering criteria.

User interface changes

See change record.

API changes

See change record.