Changed handling of wxApp, allows quit when update dialog gets closed

Instead of an idle wx related thread when all UI elements are released, WinSparkle now
shuts down the App when the update dialog gets closed (`SetExitOnFrameDelete(true)`).
The application handles `OnInit` and `OnExit` to initialize and shutdown UI related properties.
Added function callback to WinSparkle

* added enumeration for different callback signals, e.g. when showing or closing update dialog
Added code documentation to callbacks, code clean up

* code documentation added for callback function
* removed unnecessary code and reverted code
Merge branch 'feature/callback' of https://github.com/justahero/winsparkle