UI scanning progress bar

[SmiVan]

For your consideration: When scanning large memory spaces, it would be convenient to have a progress bar so the user can see that the application is not stuck and can get the general idea of how long they have to wait for.

I've made a basic example implementation of this at SmiVan/med:progressbar, but I'm a Qt beginner and don't practice C++ very often, so its probably a bit too primitive/messy for a pull request.

That being said, I think that the general idea of incrementing a mutex-guarded counter at the end of each map-scanning thread is sufficient to provide a decent progress bar without affecting the scanning speed too much.