Fix qt.qpa.services: Failed to register with host portal

[AHSauge]

The files need to match the identifier used

I need to test this a bit more, but at least on Fedora, there's no question that the desktop file needs to match the app ID, aka. it needs to be named com.github.Murmele.Gittyup.desktop otherwise I get

qt.qpa.services: Failed to register with host portal QDBusError("org.freedesktop.portal.Error.Failed", "Could not register app ID: App info not found for 'com.github.Murmele.Gittyup'")

This quite likely impacts the integration with the desktop environment, which could explain #891.

What I'm quite sceptical about is the setApplicationName in Application::Application. It sets it to GITTYUP_NAME which is Gittyup. However, in the desktop file we have StartupWMClass=com.github.Murmele.Gittyup, and if I read https://linuxvox.com/blog/what-represents-startupwmclass-entry-from-a-desktop-file-in-ubuntu/ correctly, what is set in setApplicationName should match StartupWMClass, which it clearly doesn't.