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

[bayo]

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

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.

thanks

[bayo]

You can test the double bar with http://www.tabondant.com/eng/tabs/foo-fighters/ballad-of-the-beaconsfield-miners#229000