Allow customizing grid's separator's/unit square's sizes

[eugenesvk]

I like to use a more detailed grid (10×10 or 16×9) to allow me to move precision in window sizing, but then the grid window becomes too large. The separators are also too thick (and this doesn't help me in square selection), but also the base square unit size is also unnecessary large. Would it be possible to add a config to change the size of separators and the basic square itself? Thanks!