Add Parchment Layout

[Danielku15]

alphaTab currently only has:

1. a vertical (historically named 'page') which does dynamic scaling based on the available width
2. a horizontal layout

We started to add more explicit layouting information via SystemsLayoutMode.UseModelLayout.

To have first class support of rendering precise music sheets as intended by MusicXML, Guitar Pro and Capella we should add a dedicated new layout Parchment. The parchment is a endless page with a fixed width defined by the stylesheet. e.g it might be a precise A4 page width.