Set the app's desktop file name

Set the desktop file name for the application using
QGuiApplication::setDesktopFileName [1]. This is used by
window managers/Wayland compositors like KWin to display
the proper window icon on Wayland rather than a generic
Wayland icon.

On Linux, the desktop file is searched in $XDG_DATA_DIRS (see
the desktop file spec, [2]), so the desktop file installed to
`/usr/share/applications/com.governikus.ausweisapp2.desktop`
is found this way.

[1] https://doc.qt.io/qt-6/qguiapplication.html#desktopFileName-prop
[2] https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html

src/ui/qml/UIPlugInQml.cpp

@@ -192,6 +192,7 @@ UIPlugInQml::UIPlugInQml()
 #ifndef Q_OS_ANDROID
 	QGuiApplication::setWindowIcon(mTrayIcon.getIcon());
 #endif
+	QGuiApplication::setDesktopFileName(QStringLiteral("com.governikus.ausweisapp2"));
 
 	connect(&mTrayIcon, &TrayIcon::fireShow, this, &UIPlugInQml::show);
 	connect(&mTrayIcon, &TrayIcon::fireQuit, this, [this] {