new editor draft.

•Included an algorithm for computing the size of Grid Tracks in the spec.
•Removed the property grid-cell-stacking.
•Removed the pseudo-element selector ::grid-cell.
•Defined that Grid Cells named explicitly using the grid-template property cannot be styled (with the removal of the ::grid-cell pseudo-element selector they cannot be selected).
•Removed the notion that Grid Cells could be given a display property which controls their inner layout.
•Corrected an error in the automatic placement example. All children of the form should be display:block to ensure they are valid Grid Items.
•Updated the Grid Columns and Grid Rows Properties grammar to correct an error and reduce ocurrences of <string>

Author: alexmog1