Use temp .blend file copybuffer for pose copy-paste

Uses similar way of storing temp data as object copy paste, just
uses different read entrypoint which does not modify current bmain.

This gives ability to easily copy-paste poses from one blender to
another one.

Hopefully doesn't introduce user-measurable differences.

Request from Peer here in the studio.

Reviewers: mont29

Reviewed By: mont29

Subscribers: hjalti, fsiddi

Differential Revision: https://developer.blender.org/D2229

source/blender/editors/include/ED_util.h

@@ -78,9 +78,6 @@ void undo_editmode_push(struct bContext *C, const char *name,
 
 void    undo_editmode_clear(void);
 
-/* cut-paste buffer free */
-void ED_clipboard_posebuf_free(void);
-
 /* ************** XXX OLD CRUFT WARNING ************* */
 
 void apply_keyb_grid(int shift, int ctrl, float *val, float fac1, float fac2, float fac3, int invert);