Allow god power UI icons to be changed via triggers

A very large number of maps use god powers that are modified with triggers to have a completely different effect to what the game originally had them do. However it is difficult to convey a powers function when you cannot change it's icon to another texture. This is to allow a trigger to set the icon for a displayed god power slot to a new one via triggers.

Note if the larger feature request of allowing lobby transfer and immediate play somehow makes it in then this may become obsolete.