Preferences, and Miscellaneous Improvements

I've begun phasing out the use of the ~/.maclibre folder to store data. The fetching of the XML package list is now done synchronously (NSURLConnection) and not stored anywhere. The list of installed packages is now stored at ~/Library/Preferences/com.maclibre.maclibre3.plist , where it belongs. Downloads are still stored at ~/maclibre, but I will be moving them to ~/Library/Application Support soon.

I've also made a bunch of cosmetic and other miscellaneous changes as well. The quit button now terminates the program in the correct way, for example.