Add sub-divisions to the viewport grid

### Feature description
I really love the grid system so far, but one thing I find myself longing for is some way to subdivide it, such that the "main" grid lines remain the most visible and opaque, while having the option to snap to smaller increments in between them.

This is a feature I found myself using all the time in Apps like Affinity Designer, I think it would be a great addition to Graphite! :>

### Graphite's current grid panel VS the one of affinity designer's

The issue names no files, tests, or entry points. Start by locating the viewport grid and snapping implementation, then compare its current grid settings with the requested behavior; done means supporting smaller snap increments while keeping the main grid lines more visible and opaque.

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