Update palette file and icons with many improvements ^__^

Add a title layer for sif file at the zero frame, to identify the file content
Reorder layers according to the frame number of each icon
Add more colors into palette
Cleanup/rename gradient_1 to content_x
Add a new gradient as content_4(just switched begin/end color stop) for use in some gradient layer icons
Enlarge tango color palette in palette file
Use new development snapshot of Synfig Studio as editor now, and
Some other changes I can't remember right now.