OpenMW-CS: Instance editing grid
Instance modifications are on grid when performed with the secondary edit button pressed down.
Available grid sizes and the default grid size need to be configurable via the user settings.
Each sub-mode (move, rotate, scale) needs its own grid settings.
The active grid sizes should be configurable via a context menu on the sub-mode button.
This task was originally part of the respective instance editing mode tasks, but has since been moved out of them.