add option to remove all shape keys at once (access from shape key menu on panel).

Without this there was no easy way to get a WYSIWYG copy of a mesh that had shape keys,
since removing them would adjust the mesh.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57246 954f8c5b-7b00-dc11-b283-0030488c597c