Easier adding of 3rd-party repos using a .aptr file and repoadder app

Although Ubuntu certainly has a good, rich repository full of software, from time to time, one will need to install from third-party sources, due to licensing issues, non-freeness, or the lack of that software or a new enough version in Ubuntu's official repositories. For that reason, to do so, the user will need to add that third-party repository, and then install whatever packages he needs from them. This aims to make the process of finding, adding, and installing software from third-party repositories as easy as possible by using a .aptr (apt repository) file with the repository line and the gpg key to sign it, which the user clicks and a repository adder application is launched that adds, authenticates, and reloads the repository, then displays the new software available for installation from that repository in Synaptic or Adept.