Add ability to customize Custom Execute text

[CL_eisenwr]

It would be nice to be able to customize what is shown in the NIPM installation window while a Custom Execute is running. Frequently I am calling cmd.exe to run custom actions, so the executing cmd.exe is not very helpful as that is just means to calling something else with arguments. I would be helpful to be able to give a better description to the users of what was happening during that call. I would be ok if it was just an addition to what is shown now.