Use standard locations for Tribler directories and files

[mihaic]

Instead of [user home dir]/.Tribler and [desktop]/TriblerDownloads, use standard locations.

For Linux, use Freedesktop.org standards, perhaps through PyXDG: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html http://www.freedesktop.org/wiki/Software/xdg-user-dirs/

For Mac OS, I found this after a quick search: https://developer.apple.com/library/mac/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html

For Windows, this looks like a good place to start: http://msdn.microsoft.com/en-us/library/windows/apps/hh967755.aspx