fix crash that happens about 10 seconds after gameplay screen is loaded

[elennick]

No description provided.

[elennick]

this was due to a threading/GC issue where if you dont unload content before it becomes garbage collection eligible, the game will crash when the garbage collector goes to clean it up... this wasnt an issue in XNA but is in Mono:

MonoGame/MonoGame#3196

http://stackoverflow.com/questions/32450100/opentk-graphics-graphicscontextexception-after-a-few-seconds-of-running

fixed it by having the screen manager unload content on screens that are removed from the stack