Issue 5956: Allow copying single CSS properties

Steps to reproduce

  1. Open Firebug
  2. Switch to the CSS panel
  3. Select issue5956.html from the Location Menu
  4. Right-click the background-image property of the #element rule and choose Copy Property Declaration
  5. Paste the copied string to a text editor
  6. Repeat steps 4 and 5 twice but choose Copy Property Name and Copy Property Value

Expected result