Change timestep_below_popup at runtime.

[whiteboxdev]

My project is set up like so:

Game Screen
Lose Screen
Pause Screen

The lose screen and pause screen are both semi-transparent popups which appear over the game screen. When the lose screen appears, the game screen should continue its time step. When the pause screen appears, the game screen should freeze its time step. Monarch currently does not have a function to change the timestep_below_popup settings property of an existing screen at runtime.