Automation Editor vertical quantisation

### Enhancement Summary
Much like there is vertical zoom and horizontal quantisation (snapping) of automation editor's points, there should be vertical quantisation (value snapping) of automation editor's points.

#### Justification
The most specific use case can be pitch bending for plugins that don't support in-piano roll pitch bending, wherein instead of carefully trying to ensure you bend the correct amount, there is automatic snapping. This can be done with a "Snap to multiples of x" option.
(However, sometimes we want a detuned instrument doing normal pitch bending, in which case there could be an option "Snap to multiples of x + n" where n is the detuned amount. But that is extrapolation.)
Y-axis snapping of automation editor values is a useful tool.

#### Mockup

Start by locating the automation editor implementation and the existing horizontal quantisation behavior. Trace how automation points are edited and snapped, then define the vertical value-snapping option described in the issue, including the proposed multiples-of-x behavior; done means automation values snap consistently without disrupting existing zoom or horizontal snapping.

Written by the indexing model from the issue text.