plasmaquick: don't use KWinInfo in Wayland dialogs
PlasmaCore.Dialog tests whether the dialog is spawning in a window ignoring the task manager or in a window with type NET::Dock. The second check requires KWinInfo, which is not available in Wayland, causing an error message to be printed whenever a PlasmaCore.Dialog is opened, such as in the task manager's GroupDialog.
Instead, only perform this test if we are in X11, avoiding the error message.
BUG: 500394