Allow to use bigger font for notes, and support double bar

hi,

here to patch

1) the default font for notes is very small but when we use a bigger one, numbers are overlaped and not fully displayed. this patch draw notes with transparent background but still mask the background line

![tuxguitar-fix-note-box](https://f.cloud.github.com/assets/535921/378880/ddeb9fa0-a56c-11e2-8faf-9ed423b9e171.png)

2) guitar pro format support the display of double bar as measure separator. it is a common notation very useful for readability. this patch only read/write the double bar from gp3, gp4, gp5 and display it. I do not check gpx. And i do not hack the tg format.

![measure-double-bar](https://f.cloud.github.com/assets/535921/378881/de0e2e76-a56c-11e2-8c22-67e0ecbb3961.png)

thanks