[EuiDataGrid] Optimize tabbable usage

[chandlerprall]

Currently, moving the mouse cursor across a data grid triggers multiple mouseout events, each calling tabbable on the grid to manage element tab orders. On our primary example, at 10 rows visible, this takes ~15-20ms to do. If the example grid is expanded to 100 rows, this time grows to a full second. SIEM has a grid where it takes at least twice as long.