Do not cancel old clipboard selection if it is same as the new one.
GTK applications seem to call wl_data_device::set_selection multiple times with
the same wl_data_source object, replacing it with itself. If we cancel it, they
will destroy it and the selection will be gone.
With this patch it is again possible to copy from GTK applications.
Test Plan: Patch provided by the reporter, he reported success.
Reviewed By: Plasma, romangg
Subscribers: michalsrb, romangg, graesslin, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D13535