Fix Wayland window associations

[ReillyBrogan]

Wayland uses the appId/desktopFileName window property to associate windows to the desktop launcher. 6dc728d renamed the .desktop launcher from kvirc.desktop to net.kvirc.KVIrc${CMAKE_KVIRC_VERSION_MAJOR}.desktop, however the desktopFileName window property was still being set to kvirc causing Wayland compositors to not correctly associate windows if they were behaving in a FreeDesktop specification compliant way (note that GNOME is not FreeDesktop-compliant here as they also use the StartupWMClass property from the .desktop to match with .desktop launchers as well as the filename).

On KDE Plasma 5 and 6 this would manifest in the following ways:

• The generic Wayland icon would be shown in the top left of all kvirc windows, and underneath the window when using the overview effect.
• If a user pinned the kvirc desktop file to the task manager and clicked it the created windows would not be correctly associated with the existing pinned desktop launcher and they would see duplicate icons in the task manager, one of which would be the pinned kvirc launcher and the other would be the kvirc windows.

This fixes all of those cases.

[ctrlaltca]

Thank you for the PR and the nice explanation of the problem.