Add properties for accessing the current number of columns and rows in Grid and GridView

It would be useful to find the current number of columns and rows that are laid out in a Grid and GridView.

Grid has 'rows' and 'columns' properties but these are used for setting the desired number of rows and columns, which may be different from the actual number of rows and columns if there are not enough items in the Grid.

Perhaps the solution is to add read-only "effectiveRows" and "effectiveColumns" properties to Grid and GridView (in the style of the read-only effectiveOpacity, effectiveLayoutDirection, etc.) that report the actual number of columns and rows.