Links Adaptation: Add target link by clicking on source link

* Adds the ability to add a link to a selection in the target
  html by clicking on a link in the source html.
* Provides validation of selections before adding links
* Removes code block that caused Add link but to add two links
* Preserves selection when working with links
* Provides validation of links before removal (must be editable target
  link)
* Hides Remove link control if target link is not editable

Change-Id: Ie525da7826d577ccf9a5c5c010407cd57a068fc3
Merge "Links Adaptation: Add target link by clicking on source link"