Support all alphaTab model features in alphaTex

[Danielku15]

At this point alphaTex does not support all functionalities of alphaTab. There is a variety features in the model which currently cannot be expressed in alphaTex like:

• Whammy Types
• Bend Types
• Clef Ottava
• Simile Marks
• Escape Sequences in Strings (e.g. for newlines)
• Score Metadata (instructions, notice, tab)
• Transpose
• Rasgueadao
• Legatos
• Beat Text
• Instrument Changes
• Fermata
• Harmonic Values
• Common Time Signature
• Tempo automation text
• Double Bar

The findings above were part of an early investigation to create a rountrip alphaTex exporter.

As part of this item we add support for these functionalities.