Add functionality to snap to grid
Add functionality to snap to grid

The save button is replaced by toggle grid button because now we use
external form to save and load data thus the save button is no longer
needed.

All operations such as placing, moving and scaling take effect of grid
snap.  This way, the user can easily align their figures.

Also fixed a bug that moving and scaling didn't work correctly when
the page is scrolled.

msakuta/SketchCanvas,msakuta/SketchCanvas,msakuta/SketchCanvas