Remove QT+GNOME+Wayland 'xcb' hack on fixed Qt versions

[danudey]

We implement code on GNOME desktops to force QT_QPA_PLATFORM to xcb; this works around a clipboard-related bug on GNOME+Wayland+Qt.

This bug was fixed (or worked around) in Qt 5.15.2, so we implement a version check; if the runtime Qt version is < 5.15.2, still force the workaround; otherwise, we don't need the workaround so we skip it.

We also add logging output so that users attempting to force the app not to use xcb won't get confused (as I did).

Note that the original discussion on this issue was in ksnip/ksnip#416, which indicates when this issue was fixed in Qt.