* Set the `WA_DeleteOnClose` attribute for the MainWindow.
* Handle the case where `programdata_path` exists but is empty.
* Emit a signal from `LaunchDialog` instead of using `exit()`.
* Remove `LoginThread` as it was never deleted and caused a `SIGSEGV` on exit.
* Handle termination and deletion of `SteamThread` and `ImageThread`.
* Rename `finished` to `completed`, to not override the inherited `finished` signal.
* Emit a signal from Account widget to do cleanup instead of immediately quitting.
