Update package installation checkbox properly (-ish)

I'm sorry if this is one of those issues we are not supposed to report yet, but anyway :)

I noticed that installation checkboxes are not updated properly as you scroll through the package list. Once `progress` has been set to 1.0 for some installed package, there's basically no way back to 0.0, and the cell eventually ends up being shown for a package which is not installed.

Unfortunately, I have no experience in OS X development, but this seems to fix the problem. Actually, it feels like a wrong place for this code, shouldn't this be handled somehow via existing binding?