Revert "add collision callbacks to gameobject (and hooked them up with actors)"

This reverts commit affc7316d3867322f7f56e3154adc428c63ab39a.
Revert "Revert "add collision callbacks to gameobject (and hooked them up with actors)""

This reverts commit c5e41ec6a1a83bd5f4df8fd83ad21fcb833ee139.