Cleanup: coding standards, mostly.

Also removed a silly call to `unregister_widget` in the deactivation hook. If the code isn't there, the widget isn't registered. (facepalm)
Add filter to disable `cleanup()` method
Implement singleton
Fix phpdoc
More singleton conversion
Add _deprecated_function notices to appropriate functions
Add method visibility
Add property visibility
Add property visibility
Correct fatal error introduced in 58e0467
Introduce magic getter to correct fatal error introduced in 246be76 when visibility was added to class properties
Enough work's being done to bump the version number
Clean up array creation
Widget shouldn't rely on the old global, now that we're using a proper singleton.
More coding standards tweaks
Nitpicks
Add some output formatting to the dropdown rendering
Markup output formatting for the widget
When a property isn't set or isn't available, it's better to say it's `null` than `false`.
An action for cleanup, and no silly check for class existence. Moar cleanup!