changes needed for EDL import to work again.
- add sequence.update(data=False) function.
- made some sequence vars editable.
- correct some comments.

also rename rna function sequence.getStripElem() --> strip_elem_from_frame()


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53509 954f8c5b-7b00-dc11-b283-0030488c597c