reduce amount of drawing in file_manager to improve performance

Created by: Emily

When I first pulled down Redox to play with it, the very first thing I noticed was how slow file_manager responded when in the /bin directory. I figured it might be a good first step to investigate why. It turns out that every redraw would draw the entire window, rather than just the portions that changed.

The main change here is that when redrawing to move the highlight around, it now only redraws the two affected rows: the previously lit row, and the newly lit row. Now you can now move the mouse around really fast and not have the highlighting lag behind.

I feel like a lot of the code that makes up file_manager would be at home in orbtk, I'd be interested in following this PR up by making the necessary widgets there, and porting file_manager over. It looks like Grid is almost what's needed, so perhaps not too much would actually need to be done in orbtk.