x11: Handle case where clipboard was reclaimed quickly

x11: Handle case where clipboard was reclaimed quickly

When the reply to a TARGETS request comes in, the clipboard may already
be reclaimed by the local app. Deal with that case (in an ugly way,
strictly speaking we should use a cancellable here).

This happens for example at startup when the initial TARGETS requests
have not been answered until after the main widow popped up. And if such
a window immediately claims the primary clipboard (like when the initial
focus is inside an entry), this race will happen.