SciTE tabs.
This does not allow primary selection to work between tabs as the tabs are
sharing a single Scintilla and the old primary selection is unclaimed when the
file is changed.
This fix adds a new ReceivedClipboard method which does not try to convert the
selection if received in a different format than asked for. It was the call to
gtk_selection_convert that seemed to cause the flickering. ReceivedClipboard is
only called from SelectionReceiver::ClipboardReceived so only occurs due to a
call to RequestSelection and can not occur because of a selection_received
signal.
ReceivedSelection is left with its complexity in case it is needed for some
other issue although it doesn't appear to be called for drag&drop, for example.