Added ability to paint in chunk w/ selected block

Also added an experimental INI-only setting to remove the flickering from the chunk editing display. (When painting, the flickering is very noticeable.) It's not in the menu because it works by using reflection to set DoubleBuffered (a protected property) to true from outside the class, so it might not work (as in crashes, bugs, etc.) in certain situations. Again, experimental. :-)
(The painting thing is a feature I had been wanting. If it's already there, just in a way I wasn't able to find, I won't blame you for denying the pull request.)