speed up populating GameGrid

These changes speed up how long it takes to populate the GameGrid after
clicking on a dat in the Tree.  MAME 0.155 dat was used for testing.
Before the changes population times were

windows .net = 2.75 seconds
linux mono = 23 seconds

After the changes, both now take 300-400ms.

This was done using 2 main changes.

1. Fully move populating the rows' Values to the CellFormatting function
2. Determine the total number of rows we will need and use
GameGrid.rowCount to set the row count to that.  This is much faster
then doing individual Add()'s for each row.