(Hopefully) this makes implementing updaters using external libraries easier on other platforms. To implement an updater on a new platform, create a new class that implements the pure virtual methods from `ExternalUpdater` and add code in the `UpdateChecker` initializer to initialize the new class.
To actually get automatic updates going, all that needs to happen is that `SparkleUpdater` needs to be initialized.
The rest of the functions can be connected to elements in the UI.
