Add Support for system layout customization.

[Danielku15]

Issues

Fixes #1183

Proposed changes

Adds a new option display.systemsLayout which allows telling alphaTab to use layout information embedded newly in the Score model. This information is filled from Guitar Pro 6-8 files (GPIF.xml) already.

It covers customizations of:

• systems layout defining how many bars to place in the systems (rows)
• defining a relative (page layout) or absolute (horizontal layout) scaling of the bars

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website