Added back some functionality to the sequencer

- removed static vars _last_seq, last_imagename and last_sounddir, replacing them with with vars in the "Editing" struct. didnt manage to get the active sequence to load so currently thats lost when loading.
- removed flag SEQ_ACTIVE
- Added operators cut, mute, unmute, deselect_all, select_invert, select, select_more, select_less, select_pick_linked, select_linked and borderselect.

source/blender/blenkernel/intern/sequence.c

@@ -192,8 +192,6 @@ void seq_free_editing(Editing *ed)
 
 	if(ed==NULL)
 		return;
-	
-	//XXX set_last_seq(NULL); /* clear_last_seq doesnt work, it screws up free_sequence */
 
 	SEQ_BEGIN(ed, seq) {
 		seq_free_sequence(seq);