Xpra: Ticket #2013: avoid awt clipboard spam

It just requests the targets over and over...

do_selection_request_event(<gtk.gdk.Event at 0x7f90910f7b70: GDK_SELECTION_REQUEST selection=CLIPBOARD, target=TARGETS, property=XAWT_TARGETS_OF_SELECTION:CLIPBOARD>)
target for CLIPBOARD: 'TARGETS'
do_selection_request_event(<gtk.gdk.Event at 0x7f90910f7b70: GDK_SELECTION_REQUEST selection=CLIPBOARD, target=TARGETS, property=XAWT_TARGETS_OF_SELECTION:CLIPBOARD>) target=TARGETS, selection=CLIPBOARD
do_selection_get(<GtkSelectionData at 0x7ffe41037310>, 0, 0) selection=CLIPBOARD

We should be able to cache the last value of TARGETS and re-use it when we see XAWT_TARGETS_OF_SELECTION:CLIPBOARD.