improvements to quantisation in song editor

[Rossmaxx]

Enhancement Summary

We have an option to change the quantisation in song editor in the master, but it is incomplete. The quantization is there but the grid lines do not get edited, unlike the behaviour in the piano roll where changing quantization updates the grid lines.

Justification

The absence of grid lines for quantization makes it harder to place everything at the right grid when the quantization is set to something other than 1 full bar. So the behaviour in piano roll should be brought to song editor. But there is a problem while doing that.

• the smaller grid lines will be barely visible.
  Solution : fix the zoom level to be 8* the current zoom level. Also, make the main grid (ie 1 bar line) to be a bit thicker than the others as well as make it a bit whiter.

Mockup

This mockup is just a reference for understanding what i mean. It is not 100% accurate.

[Rossmaxx]

Fixed in #7034