GC Frameplot graph

Add new graph based on the following user request:
"However, I am wondering if it would be possible to include a GC Frameplot graph (see http://www.nih.go.jp/~jun/research/frameplot/stmkan.html for example) in the DNA Graphs plugin? This graph is similar to the GC content graph but shows the GC content of the first, second and third position independently. It is very useful to identify gene starts especially in high G+C genoms and should be quite easy to implement. It is for example also implemented in the Artemis tool http://www.sanger.ac.uk/resources/software/artemis/. A publication describing the "algorithm" can be found here: http://www.ncbi.nlm.nih.gov/pubmed?term=10339816"