Piano roll grid menu

[swesterfeld]

This PR adds support for letting the user set the grid size manually. It supports tuplets (we could not do this at all before), so for instance you can uses 1/8 triplet grids. It also supports small grid sizes, while being smart enough to not draw too many grid lines.

I've preserved the code we already have for automatic grid size selection. I believe that this can be nice, because you can zoom in, and as you zoom in the grid adapts to smaller and smaller grids. So with the PR, I added an explicit choice between grid mode "auto" (automatic grid size) and "manual" (user defined grid size).