Be able to show an avarage memory usage

[Sandstedt]

I had a hard time reading the memory usage when it updated every frame, so I added an optional setting that calculates the average memory usage over a set amount of frames.

Not sure if this is the most efficient way of doing it, but it's at least flexible and it doesn't interfere with how it worked before.

Maybe other will benefit from this too, else just reject this PR and I keep it in my own fork :)