* complete re-write how we size/position items dealing with margin
* we now have a single `margin` grid option which will set all 4 sides instead of one in code nad one in CSS
* also have `marginTop/Bottom/Left/Right` if more precise setting is required
* greatly reduced the layout code as margin are now inside the grid-item
(affects content)
* fixed code so `cellHeight: 'auto'` (now the default) will create real square cells
(used to be off due to margin)
* grid will now have consistant spacing (above/below first/last rows)

* forgot to update the test case for gridstack#810 changes
* also found getCellFromPixel() was incorrect in rounding as well or delaing with margins
(which don't affect cell size, just content)