* Make functions in `main.cpp` static as they are not used by
other units
* Delete the `TrayIcon` via an extra parent object that is
deleted before the `QNetworkAccessManager` is deleted; otherwise
the destruction of `SyncthingConnection` (which aborts pending
replies) might access dangling `QNetworkReply` objects
* Improve error handling in `SingleInstance` code
* Allow to replace the current instance via the new `--replace`
argument; this may be useful when creating an installer/updater