Crash when pressing escape to cancel a model placement

[JShep1]

As found here, if the user continues to select models, most simply just a shape from the top toolbar, and presses escape to cancel the action, a crash will eventually occur.

This is likely due to these calls being made from outside the render thread. If we could have some sort of flag we set when an escape key release is detected and then have the render thread read that flag and perform the deselect and remove operations, this would probably suffice.

[JShep1]

Fixed in #493