lisp/frameset.el: Various fixes.

lisp/frameset.el: Various fixes.
(frameset-p): Don't check non-nullness of the `properties'
slot , which can indeed be nil.
(frameset-live-filter-alist, frameset-persistent-filter-alist):
Move entry for `left' from persistent to live filter alist.
(frameset-filter-alist, frameset--minibufferless-last-p, frameset-save):
Doc fixes.
(frameset-filter-params): When restoring a frame, copy items added to
`filtered', to avoid unwittingly modifying the original parameters.
(frameset-move-onscreen): Rename from frameset--move-onscreen.  Doc fix.
(frameset--restore-frame): Fix reference to frameset-move-onscreen.