effects/desktopgrid: Fix grid's size and scale w.r.t. row/column spacing

The last patch makes grid actually account for the spacing, so that when fully shown it won't cut off bottom or right edge of desktops. It can be easily seen with a wallpaper that has visible features on the edges such as borders, or by settings rowSpacing & columnSpacing to an absurdly high value.

Other commits are cleanups, type annotations, a bit of porting away from largeSpacing and a minor temporary fix for "targetScreen is undefined".