[MU4 Issue] Improvement: use "snap to grid" configuration to set keyboard nudge increment

[MarcSabatella]

Not a regression; snap to grid works as it did in 3.6.2. However, with the change to the how keyboard adjustments work (they formerly moved in 0.1 sp increments but now it's 0.5 sp, which several people have complained is too coarse), I think some simple enhancements to the snap to grid functionality could make every happy (!):

1. Use the grid configuration settings to determine the nudge increment (whether "snap to grid" is actually enabled or not)
2. Make both the drag and keyboard snap behavior apply to line handles (including tuplet brackets), not just position
3. Replace the fractional specification for the grid with a decimal one

To be clear, when I mention keyboard nudge, I'm talking about behavior when select an element and moving it directly with the keyboard, not the increment used for the offset spin box. Although, that too could be nice!