Skeleton Sketching bugfixes/tweaks

* Disabling Skeleton Sketching now refreshes the view properly, so that strokes
don't linger on even after being disabled
* Added the delete operator to the panel


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54171 954f8c5b-7b00-dc11-b283-0030488c597c