Fix missing app ID and icon on Wayland.

X11/Wayland needs to know the name of the .desktop file to show a dock
icon and application name. X11 has various means of guessing the
filename (often WM_NAME). Wayland is a bit more strict, and requires
that either the filename match the AppId or the .desktop filename be
specified.

main/main.cpp

@@ -92,6 +92,10 @@ int main(int argc, char *argv[]) {
                    &MainWindow::messageAvailable);
 #endif
 
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 7, 0))
+  QGuiApplication::setDesktopFileName("qtpass.desktop");
+#endif
+
   w.show();
 
   return SingleApplication::exec();