Usability fix: 

Blender's data link button (typically with menu and searching options)
now has a X icon to clear its contents. 

Before you had to click, delete text, enter.

For example:
- Object Parent
- Modifier objects or vertexgroups

This fix saves each user 100 clicks per day, with 100k users
that's 3 billion clicks per year!




git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53684 954f8c5b-7b00-dc11-b283-0030488c597c