Allow Bar-wise voice writing in alphaTex

[Danielku15]

Currently alphaTex only allows writing multi-voiced pieces by writing first all bars of voice 1, then all bars of voice 2 etc.

  // Voice 1
  \\voice
      // Bar 1 Voice 1
      C4*4 | 
      // Bar 2 Voice 1
      C5*4 | 
      // Bar 3 Voice 1
      C6*4
  // Voice 2
  \\voice
      // Bar 1 Voice 2
      C3*4 | 
      // Bar 2 Voice 2
      C4*4 | 
      // Bar 3 Voice 2
      C5 * 4

This generates quite a bit of overhead if you only want to add second voices in some individual cases. or when you want to edit the notation closer together this order is a bit cumbersome.

Hence it is a good addition to allow adding voices to invidual bars only.

e.g. something like:

\\voiceMode barWise
// Bar 1
    // Bar 1 Voice 1
    \\voice
    C4*4 
    // Bar 1 Voice 2
    \\voice 
    C3*4 
| 
// Bar 2
    // Bar 2 Voice 1
    \\voice
    C5*4 
    // Bar 2 Voice 2
    \\voice
    C4*4 
| 
// Bar 3
    // Bar 3 Voice 1
    \\voice
    C6*4
    // Bar 3 Voice 2
    \\voice 
    C5 * 4