Add a sub-application to Rare to launch the webview for logging into EGS.
The sub-application operates similatly to the `laucher` sub-application and
it is autonomous. After a successful login in returns the exchange code
to the standard output to be parsed and used by the login dialog.
The reason this implementation was chosen is because when pywebview uses
pyqtwebengine as the GUI library, we cannot launch it through Rare as
it tries to spawn a QMainWindow inside an existing event loop, which is
prohibited by Qt.
Despite that, EGS login page doesn't work correctly with QtWebEngine,
so on linux default to the GTK backend for pywebview, and this change
helps keeping applications using different toolkits separate.
At this moment, spawning the sub-application blocks the execution of the
main application.
This change should make it easier to authenticate through Rare inside
a gamescope session, such as the steam deck.
